LeetCode 01xx

LeetCode problems 100–199

100 items

LeetCode 01xx

# Problem Difficulty Time Notes
100 Same Tree 🟢 Easy 1m 39s The problem asks us to determine whether two binary trees are exact…
101 Symmetric Tree 🟢 Easy 1m 25s The problem asks us to determine whether a binary tree is symmetric…
102 Binary Tree Level Order Traversal 🟡 Medium 1m 30s The problem asks us to perform a level order traversal on a binary …
103 Binary Tree Zigzag Level Order Traversal 🟡 Medium 1h 33m The problem asks us to perform a level order traversal of a binary …
104 Maximum Depth of Binary Tree 🟢 Easy 2m 53s The problem asks us to determine the maximum depth of a binary tree…
105 Construct Binary Tree from Preorder and Inorder Traversal 🟡 Medium 59s The problem is asking us to reconstruct a binary tree given two tra…
106 Construct Binary Tree from Inorder and Postorder Traversal 🟡 Medium 1m 9s The problem provides two traversal orders of the same binary tree: …
107 Binary Tree Level Order Traversal II 🟡 Medium 36s The problem asks us to traverse a binary tree and return the node v…
108 Convert Sorted Array to Binary Search Tree 🟢 Easy 1m 22s The problem gives us a sorted integer array nums in strictly increa…
109 Convert Sorted List to Binary Search Tree 🟡 Medium 1m 20s The problem gives us the head of a singly linked list whose values …
110 Balanced Binary Tree 🟢 Easy 1m 11s This problem asks us to determine whether a given binary tree is he…
111 Minimum Depth of Binary Tree 🟢 Easy 37s The problem asks us to determine the minimum depth of a binary tree…
112 Path Sum 🟢 Easy 1m 9s This problem asks us to determine whether a binary tree contains at…
113 Path Sum II 🟡 Medium 1m 17s This problem asks us to find all root-to-leaf paths in a binary tre…
114 Flatten Binary Tree to Linked List 🟡 Medium 1m 11s The problem asks us to transform a binary tree into a flattened str…
115 Distinct Subsequences 🔴 Hard 1m 30s This problem asks us to count how many distinct subsequences of a s…
116 Populating Next Right Pointers in Each Node 🟡 Medium 53s This problem asks us to populate the next pointers of nodes in a pe…
117 Populating Next Right Pointers in Each Node II 🟡 Medium 1m 8s The problem asks us to populate the next pointer for every node in …
118 Pascal's Triangle 🟢 Easy 40s The problem is asking us to generate the first numRows of Pascal's …
119 Pascal's Triangle II 🟢 Easy 59s This problem asks us to return a specific row from Pascal's Triangl…
120 Triangle 🟡 Medium 1m 24s The problem gives us a triangular array of integers and asks us to …
121 Best Time to Buy and Sell Stock 🟢 Easy 56s This problem asks us to determine the maximum profit that can be ma…
122 Best Time to Buy and Sell Stock II 🟡 Medium 1m 2s This problem asks us to maximize profit from stock trading over a s…
123 Best Time to Buy and Sell Stock III 🔴 Hard 46s The problem asks us to determine the maximum profit we can earn fro…
124 Binary Tree Maximum Path Sum 🔴 Hard 49s The problem is asking us to find the maximum path sum in a binary t…
125 Valid Palindrome 🟢 Easy 53s This problem asks us to determine whether a given string is a palin…
126 Word Ladder II 🔴 Hard 1m 22s This problem asks us to find all shortest transformation sequences …
127 Word Ladder 🔴 Hard 1m 19s This problem asks us to determine the length of the shortest transf…
128 Longest Consecutive Sequence 🟡 Medium 59s The problem asks us to find the length of the longest sequence of c…
129 Sum Root to Leaf Numbers 🟡 Medium 1m 8s The problem gives us the root of a binary tree where every node con…
130 Surrounded Regions 🟡 Medium 1m 13s The problem gives us a two dimensional grid called board, where eac…
131 Palindrome Partitioning 🟡 Medium 1m 11s The problem gives us a string s and asks us to divide it into subst…
132 Palindrome Partitioning II 🔴 Hard 1m 41s The problem asks us to split a string into substrings such that eve…
133 Clone Graph 🟡 Medium 1m 27s This problem asks us to create a deep copy of an undirected connect…
134 Gas Station 🟡 Medium 1m 14s The problem describes a circular route containing n gas stations. A…
135 Candy 🔴 Hard 1m 16s This problem asks us to distribute candies to children standing in …
136 Single Number 🟢 Easy 1m 2s The problem gives an integer array nums where every value appears e…
137 Single Number II 🟡 Medium 34s This problem asks us to find a unique number in an array where ever…
138 Copy List with Random Pointer 🟡 Medium 1m 25s This problem asks us to create a completely independent copy of a l…
139 Word Break 🟡 Medium 39s The problem asks us to determine whether a given string s can be br…
140 Word Break II 🔴 Hard 49s The problem asks us to segment a given string s into all possible s…
141 Linked List Cycle 🟢 Easy 1m 12s This problem asks us to determine whether a singly linked list cont…
142 Linked List Cycle II 🟡 Medium 1m 6s This problem asks us to determine whether a singly linked list cont…
143 Reorder List 🟡 Medium 1m 34s The problem gives us the head of a singly linked list and asks us t…
144 Binary Tree Preorder Traversal 🟢 Easy 1m 35s This problem asks us to return the preorder traversal of a binary t…
145 Binary Tree Postorder Traversal 🟢 Easy 1m 26s The problem asks us to return the postorder traversal of a binary t…
146 LRU Cache 🟡 Medium 1m 24s The problem asks us to design an efficient cache that follows the L…
147 Insertion Sort List 🟡 Medium 1m 23s The problem asks us to sort a singly linked list using the insertio…
148 Sort List 🟡 Medium 1m 11s The problem asks us to sort a singly linked list in ascending order…
149 Max Points on a Line 🔴 Hard 1m 43s The problem gives us a list of points on a 2D coordinate plane. Eac…
150 Evaluate Reverse Polish Notation 🟡 Medium 1m 18s This problem asks us to evaluate an arithmetic expression written i…
151 Reverse Words in a String 🟡 Medium 33s The problem asks us to take a string s containing words separated b…
152 Maximum Product Subarray 🟡 Medium 1m 12s The problem asks us to find the contiguous subarray within an integ…
153 Find Minimum in Rotated Sorted Array 🟡 Medium 1m This problem asks us to find the smallest element in a sorted array…
154 Find Minimum in Rotated Sorted Array II 🔴 Hard 1m 1s This problem asks us to find the minimum value in a sorted array th…
155 Min Stack 🟡 Medium 1m 5s The problem asks us to design a custom stack data structure that be…
156 Binary Tree Upside Down 🟡 Medium 32s The problem asks us to transform a binary tree into its "upside-dow…
157 Read N Characters Given Read4 🟢 Easy 1m 13s This problem asks us to implement a read function using a restricte…
158 Read N Characters Given read4 II - Call Multiple Times 🔴 Hard 1m 1s This problem asks us to implement a read(buf, n) function using a r…
159 Longest Substring with At Most Two Distinct Characters 🟡 Medium 1m 12s The problem asks us to find the length of the longest contiguous su…
160 Intersection of Two Linked Lists 🟢 Easy 1m 21s This problem asks us to determine whether two singly linked lists s…
161 One Edit Distance 🟡 Medium 1m 7s The problem asks us to determine whether two strings are exactly on…
162 Find Peak Element 🟡 Medium 1m 10s The problem gives us an integer array nums and asks us to find the …
163 Missing Ranges 🟢 Easy 1m 11s This problem gives us three inputs: - A sorted array nums - A lower…
164 Maximum Gap 🟡 Medium 1m 11s The problem asks us to compute the largest difference between two c…
165 Compare Version Numbers 🟡 Medium 59s This problem asks us to compare two software version numbers repres…
166 Fraction to Recurring Decimal 🟡 Medium 1m 5s This problem asks us to convert a fraction, represented by an integ…
167 Two Sum II - Input Array Is Sorted 🟡 Medium 1m 1s This problem gives us a sorted, 1-indexed array of integers called …
168 Excel Sheet Column Title 🟢 Easy 1m 23s The problem asks us to convert a positive integer into the column n…
169 Majority Element 🟢 Easy 30s The problem asks us to identify the majority element in a given arr…
170 Two Sum III - Data structure design 🟢 Easy 1m 6s The problem asks us to design a data structure that supports two op…
171 Excel Sheet Column Number 🟢 Easy 1m 3s This problem asks us to convert an Excel-style column title into it…
172 Factorial Trailing Zeroes 🟡 Medium 1m 36s The problem asks us to determine how many trailing zeroes appear at…
173 Binary Search Tree Iterator 🟡 Medium 56s It looks like your prompt accidentally concatenated two problems. I…
174 Dungeon Game 🔴 Hard 1m 10s This problem asks us to determine the minimum initial health a knig…
175 Combine Two Tables 🟢 Easy 46s The problem is asking us to combine two relational database tables,…
176 Second Highest Salary 🟡 Medium 2m 36s This problem asks us to retrieve the second highest distinct salary…
177 Nth Highest Salary 🟡 Medium 1m 12s This problem asks us to write a SQL function that returns the nth h…
178 Rank Scores 🟡 Medium 1m 17s The problem gives us a database table named Scores that contains tw…
179 Largest Number 🟡 Medium 1m 18s This problem asks us to rearrange a list of non-negative integers s…
180 Consecutive Numbers 🟡 Medium 41s This problem asks us to find numbers in a table that appear at leas…
181 Employees Earning More Than Their Managers 🟢 Easy 1m 15s This problem gives us a single database table named Employee. Each …
182 Duplicate Emails 🟢 Easy 2m 51s The problem gives us a database table named Person with two columns…
183 Customers Who Never Order 🟢 Easy 31s This problem is asking us to identify all customers from a Customer…
184 Department Highest Salary 🟡 Medium 39s The problem asks us to find employees with the highest salary in ea…
185 Department Top Three Salaries 🔴 Hard 1m 12s This is a SQL database problem where we need to identify the employ…
186 Reverse Words in a String II 🟡 Medium 39s The problem asks us to reverse the order of words in a character ar…
187 Repeated DNA Sequences 🟡 Medium 53s This problem asks us to find all repeated DNA subsequences of a fix…
188 Best Time to Buy and Sell Stock IV 🔴 Hard 39s This problem asks us to maximize the profit from stock trading unde…
189 Rotate Array 🟡 Medium 50s The problem asks us to rotate a given integer array nums to the rig…
190 Reverse Bits 🟢 Easy 56s The problem asks us to reverse the binary representation of a 32-bi…
191 Number of 1 Bits 🟢 Easy 53s The problem asks us to count how many bits are set to 1 in the bina…
192 Word Frequency 🟡 Medium 2m 21s This problem asks us to write a shell script that reads a text file…
193 Valid Phone Numbers 🟢 Easy 1m 27s This problem asks us to process a text file named file.txt and prin…
194 Transpose File 🟡 Medium 1m 6s This problem asks us to transpose the contents of a text file. Tran…
195 Tenth Line 🟢 Easy 52s This problem asks us to print exactly the 10th line from a text fil…
196 Delete Duplicate Emails 🟢 Easy 52s This problem provides a database table named Person with two column…
197 Rising Temperature 🟢 Easy 3m 42s The problem gives us a database table named Weather that stores dai…
198 House Robber 🟡 Medium 3m 4s The problem describes a row of houses, where each house contains so…
199 Binary Tree Right Side View 🟡 Medium 1m 34s The problem gives us the root node of a binary tree and asks us to …
Practice › LeetCode › LeetCode 01xx ›

LeetCode 100 - Same Tree

The problem asks us to determine whether two binary trees are exactly the same. We are given the roots of two binary trees, p and q. A binary tree consists of nodes where each node contains a value and references to a left child and a right child.
leetcode Easy tree depth-first-search breadth-first-search binary-tree May 28, 2026
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LeetCode 101 - Symmetric Tree

The problem asks us to determine whether a binary tree is symmetric around its center. A tree is symmetric if the left subtree is a mirror reflection of the right subtree.
leetcode Easy tree depth-first-search breadth-first-search binary-tree May 28, 2026
Practice › LeetCode › LeetCode 01xx ›

LeetCode 102 - Binary Tree Level Order Traversal

The problem asks us to perform a level order traversal on a binary tree. A binary tree is a hierarchical structure where each node can have at most two children, commonly referred to as the left child and the right child.
leetcode Medium tree breadth-first-search binary-tree May 28, 2026
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LeetCode 103 - Binary Tree Zigzag Level Order Traversal

The problem asks us to perform a level order traversal of a binary tree, but with a twist. Instead of always traversing each level from left to right, we alternate the traversal direction at every level.
leetcode Medium tree breadth-first-search binary-tree May 28, 2026
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LeetCode 104 - Maximum Depth of Binary Tree

The problem asks us to determine the maximum depth of a binary tree. A binary tree consists of nodes where each node can have up to two children, a left child and a right child.
leetcode Easy tree depth-first-search breadth-first-search binary-tree May 28, 2026
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LeetCode 105 - Construct Binary Tree from Preorder and Inorder Traversal

The problem is asking us to reconstruct a binary tree given two traversal orders: preorder and inorder. In a preorder traversal, nodes are visited in the order: root, left subtree, right subtree.
leetcode Medium array hash-table divide-and-conquer tree binary-tree May 28, 2026
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LeetCode 106 - Construct Binary Tree from Inorder and Postorder Traversal

The problem provides two traversal orders of the same binary tree: - inorder, which follows the order: left subtree, root, right subtree - postorder, which follows the order: left subtree, right subtree, root We must reconstruct the original binary tree and return its root node.
leetcode Medium array hash-table divide-and-conquer tree binary-tree May 28, 2026
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LeetCode 107 - Binary Tree Level Order Traversal II

The problem asks us to traverse a binary tree and return the node values level by level, but in bottom-up order. Normally, a level order traversal collects values from the root down to the leaves.
leetcode Medium tree breadth-first-search binary-tree May 28, 2026
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LeetCode 108 - Convert Sorted Array to Binary Search Tree

The problem gives us a sorted integer array nums in strictly increasing order and asks us to convert it into a height-balanced binary search tree (BST). A binary search tree is a binary tree where: - Every node in the left subtree contains values smaller than the current node.
leetcode Easy array divide-and-conquer tree binary-search-tree binary-tree May 28, 2026
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LeetCode 109 - Convert Sorted List to Binary Search Tree

The problem gives us the head of a singly linked list whose values are already sorted in ascending order. We must convert this linked list into a height balanced Binary Search Tree, usually abbreviated as BST.
leetcode Medium linked-list divide-and-conquer tree binary-search-tree binary-tree May 28, 2026
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LeetCode 110 - Balanced Binary Tree

This problem asks us to determine whether a given binary tree is height-balanced. A binary tree is considered height-balanced if, for every node in the tree, the height difference between its left and right subtrees is at most 1.
leetcode Easy tree depth-first-search binary-tree May 28, 2026
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LeetCode 111 - Minimum Depth of Binary Tree

The problem asks us to determine the minimum depth of a binary tree. In other words, given the root of a binary tree, we want to find the shortest path from the root node down to the nearest leaf node.
leetcode Easy tree depth-first-search breadth-first-search binary-tree May 28, 2026
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LeetCode 112 - Path Sum

This problem asks us to determine whether a binary tree contains at least one valid root-to-leaf path whose node values add up to a given target sum. A binary tree consists of nodes where each node can have a left child and a right child. Each node also stores an integer value.
leetcode Easy tree depth-first-search breadth-first-search binary-tree May 28, 2026
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LeetCode 113 - Path Sum II

This problem asks us to find all root-to-leaf paths in a binary tree where the sum of the node values equals a given targetSum. A binary tree is provided through the root node, and every node contains an integer value. We are also given an integer targetSum.
leetcode Medium backtracking tree depth-first-search binary-tree May 28, 2026
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LeetCode 114 - Flatten Binary Tree to Linked List

The problem asks us to transform a binary tree into a flattened structure that behaves like a singly linked list. The transformation must happen in-place, meaning we are not supposed to create an entirely new tree or list structure.
leetcode Medium linked-list stack tree depth-first-search binary-tree May 28, 2026
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LeetCode 115 - Distinct Subsequences

This problem asks us to count how many distinct subsequences of a string s are equal to another string t. A subsequence is formed by deleting zero or more characters from a string without changing the relative order of the remaining characters.
leetcode Hard string dynamic-programming May 28, 2026
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LeetCode 116 - Populating Next Right Pointers in Each Node

This problem asks us to populate the next pointers of nodes in a perfect binary tree. A perfect binary tree has all levels completely filled; every internal node has exactly two children, and all leaves are at the same depth.
leetcode Medium linked-list tree depth-first-search breadth-first-search binary-tree May 28, 2026
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LeetCode 117 - Populating Next Right Pointers in Each Node II

The problem asks us to populate the next pointer for every node in a binary tree so that it points to the node immediately to its right on the same level. If no such node exists, the next pointer should remain NULL.
leetcode Medium linked-list tree depth-first-search breadth-first-search binary-tree May 28, 2026
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LeetCode 118 - Pascal's Triangle

The problem is asking us to generate the first numRows of Pascal's triangle, which is a triangular arrangement of numbers where each number is the sum of the two numbers directly above it.
leetcode Easy array dynamic-programming May 28, 2026
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LeetCode 119 - Pascal's Triangle II

This problem asks us to return a specific row from Pascal's Triangle. The input, rowIndex, represents the zero-based index of the row we want to generate.
leetcode Easy array dynamic-programming May 28, 2026
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LeetCode 120 - Triangle

The problem gives us a triangular array of integers and asks us to compute the minimum path sum from the top row to the bottom row. The input is a two-dimensional array called triangle, where: - The first row contains exactly one number.
leetcode Medium array dynamic-programming May 28, 2026
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LeetCode 121 - Best Time to Buy and Sell Stock

This problem asks us to determine the maximum profit that can be made from a single stock transaction. A transaction consists of buying one stock on one day and selling it on a later day. The key restriction is that the selling day must come after the buying day.
leetcode Easy array dynamic-programming May 28, 2026
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LeetCode 122 - Best Time to Buy and Sell Stock II

This problem asks us to maximize profit from stock trading over a sequence of days. We are given an array prices, where prices[i] represents the stock price on the ith day.
leetcode Medium array dynamic-programming greedy May 28, 2026
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LeetCode 123 - Best Time to Buy and Sell Stock III

The problem asks us to determine the maximum profit we can earn from at most two stock transactions given the daily prices of a stock in an array prices. A transaction is defined as buying once and selling once.
leetcode Hard array dynamic-programming May 28, 2026
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LeetCode 124 - Binary Tree Maximum Path Sum

The problem is asking us to find the maximum path sum in a binary tree. A path is defined as any sequence of nodes connected by edges, where each node is included at most once. The path does not need to start at the root or end at a leaf.
leetcode Hard dynamic-programming tree depth-first-search binary-tree May 28, 2026
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LeetCode 125 - Valid Palindrome

This problem asks us to determine whether a given string is a palindrome after applying two transformations: 1. Convert all uppercase letters to lowercase. 2. Remove all non-alphanumeric characters. An alphanumeric character is any English letter (a-z, A-Z) or digit (0-9).
leetcode Easy two-pointers string May 28, 2026
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LeetCode 126 - Word Ladder II

This problem asks us to find all shortest transformation sequences from beginWord to endWord, where each transformation changes exactly one character and every intermediate word must exist in wordList. We can think of the problem as navigating through a graph.
leetcode Hard hash-table string backtracking breadth-first-search May 28, 2026
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LeetCode 127 - Word Ladder

This problem asks us to determine the length of the shortest transformation sequence between two words, beginWord and endWord, under a strict transformation rule.
leetcode Hard hash-table string breadth-first-search May 28, 2026
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LeetCode 128 - Longest Consecutive Sequence

The problem asks us to find the length of the longest sequence of consecutive integers in an unsorted array. A consecutive sequence means numbers that appear one after another numerically, regardless of their position in the array.
leetcode Medium array hash-table union-find May 28, 2026
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LeetCode 129 - Sum Root to Leaf Numbers

The problem gives us the root of a binary tree where every node contains a single digit from 0 to 9. Every path starting from the root and ending at a leaf node forms a number by concatenating the digits along that path.
leetcode Medium tree depth-first-search binary-tree May 28, 2026
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LeetCode 130 - Surrounded Regions

The problem gives us a two dimensional grid called board, where each cell contains either 'X' or 'O'. The goal is to modify the board in place by capturing every region of 'O' cells that is completely surrounded by 'X'.
leetcode Medium array depth-first-search breadth-first-search union-find matrix May 28, 2026
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LeetCode 131 - Palindrome Partitioning

The problem gives us a string s and asks us to divide it into substrings such that every substring is a palindrome. A palindrome is a string that reads the same forward and backward. We must return all possible valid ways to partition the string.
leetcode Medium string dynamic-programming backtracking May 28, 2026
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LeetCode 132 - Palindrome Partitioning II

The problem asks us to split a string into substrings such that every substring is a palindrome. Among all valid palindrome partitions, we must return the minimum number of cuts required. A cut divides the string into two parts.
leetcode Hard string dynamic-programming May 28, 2026
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LeetCode 133 - Clone Graph

This problem asks us to create a deep copy of an undirected connected graph. We are given a reference to one node in the graph, and we must return a completely independent copy of the entire graph structure.
leetcode Medium hash-table depth-first-search breadth-first-search graph-theory May 28, 2026
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LeetCode 134 - Gas Station

The problem describes a circular route containing n gas stations. At each station i, you can collect gas[i] units of fuel. Traveling from station i to station (i + 1) % n consumes cost[i] units of fuel.
leetcode Medium array greedy May 28, 2026
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LeetCode 135 - Candy

This problem asks us to distribute candies to children standing in a line, based on their rating values. Each child must receive at least one candy, and any child with a higher rating than an adjacent neighbor must receive strictly more candies than that neighbor.
leetcode Hard array greedy May 28, 2026
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LeetCode 136 - Single Number

The problem gives an integer array nums where every value appears exactly twice except for one value that appears only once. The task is to find and return that unique value.
leetcode Easy array bit-manipulation May 28, 2026
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LeetCode 137 - Single Number II

This problem asks us to find a unique number in an array where every other number appears exactly three times. In simpler terms, if you imagine counting all the numbers in the array, every number except one will show up three times, and our task is to identify the number that…
leetcode Medium array bit-manipulation May 28, 2026
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LeetCode 138 - Copy List with Random Pointer

This problem asks us to create a completely independent copy of a linked list where each node contains two pointers: - next, which points to the next node in the list - random, which can point to any node in the list or null The key requirement is that the copied list must be…
leetcode Medium hash-table linked-list May 28, 2026
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LeetCode 139 - Word Break

The problem asks us to determine whether a given string s can be broken into a sequence of valid words from a given dictionary wordDict. Essentially, we need to check if we can insert spaces into s such that every substring separated by these spaces exists in the dictionary.
leetcode Medium array hash-table string dynamic-programming trie memoization May 28, 2026
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LeetCode 140 - Word Break II

The problem asks us to segment a given string s into all possible sentences where each word in the sentence exists in a given dictionary wordDict.
leetcode Hard array hash-table string dynamic-programming backtracking trie memoization May 28, 2026
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LeetCode 141 - Linked List Cycle

This problem asks us to determine whether a singly linked list contains a cycle. A cycle exists when following the next pointers eventually leads back to a node that has already been visited, instead of reaching None. The input is the head node of a singly linked list.
leetcode Easy hash-table linked-list two-pointers May 28, 2026
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LeetCode 142 - Linked List Cycle II

This problem asks us to determine whether a singly linked list contains a cycle and, if it does, return the exact node where that cycle begins. A linked list is normally a sequence of nodes where each node points to the next one, eventually ending with null.
leetcode Medium hash-table linked-list two-pointers May 28, 2026
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LeetCode 143 - Reorder List

The problem gives us the head of a singly linked list and asks us to reorder the nodes in a very specific alternating pattern. A normal linked list looks like this: We must transform it into: The important detail is that we are not allowed to change node values.
leetcode Medium linked-list two-pointers stack recursion May 28, 2026
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LeetCode 144 - Binary Tree Preorder Traversal

This problem asks us to return the preorder traversal of a binary tree. A binary tree consists of nodes where each node may have a left child and a right child. The input root represents the root node of that tree.
leetcode Easy stack tree depth-first-search binary-tree May 28, 2026
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LeetCode 145 - Binary Tree Postorder Traversal

The problem asks us to return the postorder traversal of a binary tree. A binary tree is made of nodes, where each node contains: - A value - A pointer to a left child - A pointer to a right child In postorder traversal, we must visit nodes in this exact order: 1.
leetcode Easy stack tree depth-first-search binary-tree May 28, 2026
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LeetCode 146 - LRU Cache

The problem asks us to design an efficient cache that follows the Least Recently Used, or LRU, eviction policy. A cache stores a limited number of key-value pairs.
leetcode Medium hash-table linked-list design doubly-linked-list May 28, 2026
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LeetCode 147 - Insertion Sort List

The problem asks us to sort a singly linked list using the insertion sort algorithm. Unlike array sorting problems where elements can be accessed directly by index, linked lists require sequential traversal, so insertion operations behave differently and require careful…
leetcode Medium linked-list sorting May 28, 2026
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LeetCode 148 - Sort List

The problem asks us to sort a singly linked list in ascending order and return the head of the sorted list. The input is the head node of a linked list. Each node contains an integer value and a pointer to the next node.
leetcode Medium linked-list two-pointers divide-and-conquer sorting merge-sort May 28, 2026
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LeetCode 149 - Max Points on a Line

The problem gives us a list of points on a 2D coordinate plane. Each point is represented as [x, y], where x and y are integer coordinates. Our task is to determine the maximum number of points that lie on the same straight line.
leetcode Hard array hash-table math geometry May 28, 2026
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LeetCode 150 - Evaluate Reverse Polish Notation

This problem asks us to evaluate an arithmetic expression written in Reverse Polish Notation, also called postfix notation. Instead of placing operators between operands as in standard infix notation, Reverse Polish Notation places operators after their operands.
leetcode Medium array math stack May 28, 2026
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LeetCode 151 - Reverse Words in a String

The problem asks us to take a string s containing words separated by spaces and return a new string where the words appear in reverse order. A word is defined as any sequence of non-space characters, and words can be separated by multiple spaces.
leetcode Medium two-pointers string May 28, 2026
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LeetCode 152 - Maximum Product Subarray

The problem asks us to find the contiguous subarray within an integer array nums that produces the largest possible product. Unlike the classic maximum subarray sum problem, multiplication introduces additional complexity because negative numbers can completely change the result.
leetcode Medium array dynamic-programming May 28, 2026
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LeetCode 153 - Find Minimum in Rotated Sorted Array

This problem asks us to find the smallest element in a sorted array that has been rotated some number of times. A sorted array in ascending order might originally look like this: After rotation, it could become: or remain unchanged: The important observation is that the array…
leetcode Medium array binary-search May 28, 2026
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LeetCode 154 - Find Minimum in Rotated Sorted Array II

This problem asks us to find the minimum value in a sorted array that has been rotated, while also allowing duplicate values. A rotated sorted array is created by taking an ascending sorted array and shifting some suffix of the array to the front.
leetcode Hard array binary-search May 28, 2026
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LeetCode 155 - Min Stack

The problem asks us to design a custom stack data structure that behaves like a normal stack, while also supporting an additional operation called getMin(). This operation must return the minimum value currently stored in the stack, and it must do so in constant time, O(1).
leetcode Medium stack design May 28, 2026
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LeetCode 156 - Binary Tree Upside Down

The problem asks us to transform a binary tree into its "upside-down" version. In more precise terms, we are given a binary tree where every right node either has a left sibling or is absent, and no right node has children.
leetcode Medium tree depth-first-search binary-tree May 28, 2026
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LeetCode 157 - Read N Characters Given Read4

This problem asks us to implement a read function using a restricted API called read4. The file itself is hidden from us, and the only way to access its contents is by repeatedly calling read4.
leetcode Easy array simulation interactive May 28, 2026
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LeetCode 158 - Read N Characters Given read4 II - Call Multiple Times

This problem asks us to implement a read(buf, n) function using a restricted API called read4(buf4). The challenge is that we are not allowed to access the file directly, instead, we can only retrieve data in chunks of up to four characters at a time.
leetcode Hard array simulation interactive May 28, 2026
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LeetCode 159 - Longest Substring with At Most Two Distinct Characters

The problem asks us to find the length of the longest contiguous substring in a string s that contains at most two distinct characters. A substring is a continuous sequence of characters inside the original string.
leetcode Medium hash-table string sliding-window May 28, 2026
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LeetCode 160 - Intersection of Two Linked Lists

This problem asks us to determine whether two singly linked lists share a common node, and if they do, return the exact node where the intersection begins. The important detail is that an intersection is based on node reference, not node value.
leetcode Easy hash-table linked-list two-pointers May 28, 2026
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LeetCode 161 - One Edit Distance

The problem asks us to determine whether two strings are exactly one edit apart. An edit can be one of three operations: 1. Insert a single character 2. Delete a single character 3. Replace one character with a different character The key detail is the word exactly.
leetcode Medium two-pointers string May 28, 2026
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LeetCode 162 - Find Peak Element

The problem gives us an integer array nums and asks us to find the index of any peak element. A peak element is defined as an element that is strictly greater than its immediate neighbors.
leetcode Medium array binary-search May 28, 2026
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LeetCode 163 - Missing Ranges

This problem gives us three inputs: - A sorted array nums - A lower bound lower - An upper bound upper The array contains unique integers, and every number in the array lies within the inclusive range [lower, upper].
leetcode Easy array May 28, 2026
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LeetCode 164 - Maximum Gap

The problem asks us to compute the largest difference between two consecutive elements after sorting the array. At first glance, this may seem straightforward: sort the array, then scan through adjacent pairs to find the maximum difference.
leetcode Medium array sorting bucket-sort radix-sort May 27, 2026
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LeetCode 165 - Compare Version Numbers

This problem asks us to compare two software version numbers represented as strings. Each version consists of one or more numeric revisions separated by dots (.). For example, "1.2.10" contains three revisions: 1, 2, and 10.
leetcode Medium two-pointers string May 28, 2026
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LeetCode 166 - Fraction to Recurring Decimal

This problem asks us to convert a fraction, represented by an integer numerator and an integer denominator, into its decimal string representation. If the decimal expansion contains a repeating fractional sequence, we must enclose the repeating part in parentheses.
leetcode Medium hash-table math string May 28, 2026
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LeetCode 167 - Two Sum II - Input Array Is Sorted

This problem gives us a sorted, 1-indexed array of integers called numbers and a target integer target. Our task is to find exactly two numbers in the array whose sum equals target, then return their 1-based indices.
leetcode Medium array two-pointers binary-search May 28, 2026
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LeetCode 168 - Excel Sheet Column Title

The problem asks us to convert a positive integer into the column naming format used by Microsoft Excel. Excel labels columns alphabetically instead of numerically. The sequence begins as: The input, columnNumber, represents a 1-based column index.
leetcode Easy math string May 28, 2026
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LeetCode 169 - Majority Element

The problem asks us to identify the majority element in a given array of integers nums. The majority element is defined as the number that appears more than half of the times in the array, i.e., more than ⌊n / 2⌋ times where n is the length of the array.
leetcode Easy array hash-table divide-and-conquer sorting counting May 28, 2026
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LeetCode 170 - Two Sum III - Data structure design

The problem asks us to design a data structure that supports two operations efficiently over a stream of integers. The first operation, add(number), inserts a number into the data structure. Numbers may appear multiple times, so duplicates must be handled correctly.
leetcode Easy array hash-table two-pointers design data-stream May 28, 2026
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LeetCode 171 - Excel Sheet Column Number

This problem asks us to convert an Excel-style column title into its corresponding numerical index. In Excel spreadsheets, columns are labeled alphabetically: The labeling system works similarly to a positional number system, except instead of digits 0-9, it uses letters A-Z…
leetcode Easy math string May 28, 2026
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LeetCode 172 - Factorial Trailing Zeroes

The problem asks us to determine how many trailing zeroes appear at the end of n!, where n! represents the factorial of n. A factorial is defined as: For example: - 5!
leetcode Medium math May 28, 2026
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LeetCode 173 - Binary Search Tree Iterator

It looks like your prompt accidentally concatenated two problems. I will answer the second one, LeetCode 173 - Binary Search Tree Iterator, following your required format.
leetcode Medium stack tree design binary-search-tree binary-tree iterator May 28, 2026
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LeetCode 174 - Dungeon Game

This problem asks us to determine the minimum initial health a knight needs in order to safely travel through a dungeon and rescue a princess.
leetcode Hard array dynamic-programming matrix May 28, 2026
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LeetCode 175 - Combine Two Tables

The problem is asking us to combine two relational database tables, Person and Address, in such a way that we report each person's first name, last name, city, and state.
leetcode Easy database May 28, 2026
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LeetCode 176 - Second Highest Salary

This problem asks us to retrieve the second highest distinct salary from the Employee table. The key word here is distinct. We are not looking for the second row after sorting salaries, we are looking for the second unique salary value.
leetcode Medium database May 28, 2026
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LeetCode 177 - Nth Highest Salary

This problem asks us to write a SQL function that returns the nth highest distinct salary from the Employee table. The table contains two columns: Column Meaning --- --- id Unique employee identifier salary Employee salary The key detail is that the salary must be distinct.
leetcode Medium database May 28, 2026
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LeetCode 178 - Rank Scores

The problem gives us a database table named Scores that contains two columns: id and score. Each row represents the score achieved in a game.
leetcode Medium database May 28, 2026
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LeetCode 179 - Largest Number

This problem asks us to rearrange a list of non-negative integers so that, when concatenated together, they form the largest possible number. At first glance, this may look like a straightforward sorting problem where we simply sort the numbers in descending order.
leetcode Medium array string greedy sorting May 28, 2026
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LeetCode 180 - Consecutive Numbers

This problem asks us to find numbers in a table that appear at least three times consecutively in order of their id. The table Logs consists of two columns: id and num, where id is an auto-incrementing primary key and num is a string representing a number.
leetcode Medium database May 28, 2026
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LeetCode 181 - Employees Earning More Than Their Managers

This problem gives us a single database table named Employee. Each row represents one employee and contains four pieces of information: Column Meaning --- --- id Unique identifier for the employee name Employee name salary Employee salary managerId The id of that employee's…
leetcode Easy database May 28, 2026
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LeetCode 182 - Duplicate Emails

The problem gives us a database table named Person with two columns: Column Description --- --- id A unique integer identifier for each row email The email address associated with that row The goal is to find all email addresses that appear more than once in the table.
leetcode Easy database May 28, 2026
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LeetCode 183 - Customers Who Never Order

This problem is asking us to identify all customers from a Customers table who have never placed an order according to the Orders table. In other words, we are looking for entries in Customers that do not have a corresponding customerId in the Orders table.
leetcode Easy database May 28, 2026
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LeetCode 184 - Department Highest Salary

The problem asks us to find employees with the highest salary in each department from a company database. The input consists of two relational tables: Employee and Department. The Employee table contains employee details, including id, name, salary, and departmentId.
leetcode Medium database May 28, 2026
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LeetCode 185 - Department Top Three Salaries

This is a SQL database problem where we need to identify the employees who belong to the top three unique salaries within each department.
leetcode Hard database May 28, 2026
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LeetCode 186 - Reverse Words in a String II

The problem asks us to reverse the order of words in a character array s in-place. Each word is a contiguous sequence of non-space characters, and words are separated by exactly one space.
leetcode Medium two-pointers string May 28, 2026
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LeetCode 187 - Repeated DNA Sequences

This problem asks us to find all repeated DNA subsequences of a fixed length, specifically length 10, inside a given DNA string. The input is a string s, where each character represents a DNA nucleotide.
leetcode Medium hash-table string bit-manipulation sliding-window rolling-hash hash-function May 28, 2026
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LeetCode 188 - Best Time to Buy and Sell Stock IV

This problem asks us to maximize the profit from stock trading under a strict constraint: we can perform at most k transactions, where each transaction consists of buying and then later selling one share of the stock.
leetcode Hard array dynamic-programming May 28, 2026
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LeetCode 189 - Rotate Array

The problem asks us to rotate a given integer array nums to the right by k steps. In other words, each element of the array should be shifted k positions forward, and the elements that "fall off" the end wrap around to the front of the array.
leetcode Medium array math two-pointers May 28, 2026
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LeetCode 190 - Reverse Bits

The problem asks us to reverse the binary representation of a 32-bit integer. Instead of reversing the decimal digits of a number, we reverse the order of its individual bits.
leetcode Easy divide-and-conquer bit-manipulation May 27, 2026
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LeetCode 191 - Number of 1 Bits

The problem asks us to count how many bits are set to 1 in the binary representation of a positive integer n. A set bit is simply a bit whose value is 1. For example, the number 11 in binary is: This binary representation contains three 1 bits, so the answer is 3.
leetcode Easy divide-and-conquer bit-manipulation May 27, 2026
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LeetCode 192 - Word Frequency

This problem asks us to write a shell script that reads a text file named words.txt and computes how many times each word appears. After counting the occurrences, the script must print every unique word together with its frequency, sorted in descending order of frequency.
leetcode Medium shell May 27, 2026
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LeetCode 193 - Valid Phone Numbers

This problem asks us to process a text file named file.txt and print only the phone numbers that match one of two valid formats. The valid formats are: and In both formats, every x represents a single digit from 0 to 9.
leetcode Easy shell May 27, 2026
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LeetCode 194 - Transpose File

This problem asks us to transpose the contents of a text file. Transposing means converting rows into columns and columns into rows. Every word in the input file is separated by a single space, and each row contains the same number of columns. The input is a file named file.txt.
leetcode Medium shell May 27, 2026
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LeetCode 195 - Tenth Line

This problem asks us to print exactly the 10th line from a text file named file.txt. The file contains multiple lines of plain text, and each line is separated by a newline character. The input is not provided as function arguments like many algorithm problems.
leetcode Easy shell May 27, 2026
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LeetCode 196 - Delete Duplicate Emails

This problem provides a database table named Person with two columns: The id column is unique because it is the primary key. The email column may contain duplicate values, meaning multiple rows can share the same email address.
leetcode Easy database May 27, 2026
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LeetCode 197 - Rising Temperature

The problem gives us a database table named Weather that stores daily temperature records. Each row contains three fields: a unique id, a recordDate, and the temperature recorded on that date.
leetcode Easy database May 27, 2026
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LeetCode 198 - House Robber

The problem describes a row of houses, where each house contains some amount of money. You are acting as a robber who wants to maximize the total amount stolen, but there is one important restriction, you cannot rob two adjacent houses.
leetcode Medium array dynamic-programming May 27, 2026
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LeetCode 199 - Binary Tree Right Side View

The problem gives us the root node of a binary tree and asks us to determine which nodes are visible when looking at the tree from the right side.
leetcode Medium tree depth-first-search breadth-first-search binary-tree May 27, 2026