LeetCode 01xx
LeetCode practice notes for problems 100 through 199, including A detailed guide to solving Same Tree with recursive DFS and structural comparison.
LeetCode 01xx
| # | Title | Difficulty | Description |
|---|---|---|---|
| 100 | LeetCode 100: Same Tree | Easy | A detailed guide to solving Same Tree with recursive DFS and structural comparison. |
| 101 | LeetCode 101: Symmetric Tree | Easy | A clear explanation of checking whether a binary tree is symmetric using mirror recursion. |
| 102 | LeetCode 102: Binary Tree Level Order Traversal | Medium | A clear explanation of binary tree level order traversal using breadth-first search and a queue. |
| 103 | LeetCode 103: Binary Tree Zigzag Level Order Traversal | Medium | A clear explanation of zigzag level order traversal using breadth-first search and alternating level direction. |
| 104 | LeetCode 104: Maximum Depth of Binary Tree | Easy | A clear explanation of finding the maximum depth of a binary tree using recursive depth-first search. |
| 105 | LeetCode 105: Construct Binary Tree from Preorder and Inorder Traversal | Medium | A clear explanation of rebuilding a binary tree from preorder and inorder traversals using recursion and an index map. |
| 106 | LeetCode 106: Construct Binary Tree from Inorder and Postorder Traversal | Medium | A clear explanation of rebuilding a binary tree from inorder and postorder traversals using recursion and an index map. |
| 107 | LeetCode 107: Binary Tree Level Order Traversal II | Medium | A clear explanation of returning binary tree levels from bottom to top using breadth-first search. |
| 108 | LeetCode 108: Convert Sorted Array to Binary Search Tree | Easy | A clear explanation of building a height-balanced binary search tree from a sorted array using divide and conquer. |
| 109 | LeetCode 109: Convert Sorted List to Binary Search Tree | Medium | A clear explanation of converting a sorted linked list into a height-balanced binary search tree using slow and fast pointers. |
| 110 | LeetCode 110: Balanced Binary Tree | Easy | A clear explanation of checking whether a binary tree is height-balanced using bottom-up depth-first search. |
| 111 | LeetCode 111: Minimum Depth of Binary Tree | Easy | A clear explanation of finding the minimum depth of a binary tree using breadth-first search. |
| 112 | LeetCode 112: Path Sum | Easy | A clear explanation of checking whether a binary tree has a root-to-leaf path whose values add up to a target sum. |
| 113 | LeetCode 113: Path Sum II | Medium | A clear explanation of finding all root-to-leaf paths whose values add up to a target sum using depth-first search and backtracking. |
| 114 | LeetCode 114: Flatten Binary Tree to Linked List | Medium | A clear explanation of flattening a binary tree into a linked list in preorder traversal order using recursive depth-first search. |
| 115 | LeetCode 115: Distinct Subsequences | Hard | A clear explanation of counting distinct subsequences using dynamic programming. |
| 116 | LeetCode 116: Populating Next Right Pointers in Each Node | Medium | A clear explanation of connecting next pointers in a perfect binary tree using constant extra space. |
| 117 | LeetCode 117: Populating Next Right Pointers in Each Node II | Medium | A clear explanation of connecting next pointers in any binary tree using constant extra space. |
| 118 | LeetCode 118: Pascal's Triangle | Easy | A clear explanation of generating Pascal's Triangle row by row using dynamic programming. |
| 119 | LeetCode 119: Pascal's Triangle II | Easy | A clear explanation of generating a single row of Pascal's Triangle using in-place dynamic programming. |
| 120 | LeetCode 120: Triangle | Medium | A clear explanation of finding the minimum path sum in a triangle using bottom-up dynamic programming. |
| 121 | LeetCode 121: Best Time to Buy and Sell Stock | Easy | A clear explanation of finding the maximum profit from one stock transaction using a single pass. |
| 122 | LeetCode 122: Best Time to Buy and Sell Stock II | Medium | A clear explanation of maximizing stock profit with unlimited transactions using a greedy single-pass method. |
| 123 | LeetCode 123: Best Time to Buy and Sell Stock III | Hard | A clear explanation of maximizing stock profit with at most two transactions using dynamic programming. |
| 124 | LeetCode 124: Binary Tree Maximum Path Sum | Hard | A clear explanation of finding the maximum path sum in a binary tree using bottom-up depth-first search. |
| 125 | LeetCode 125: Valid Palindrome | Easy | A clear explanation of checking whether a string is a palindrome after ignoring non-alphanumeric characters and case. |
| 126 | LeetCode 126: Word Ladder II | Hard | Find all shortest word transformation sequences using BFS to build shortest-path parents, then backtracking to reconstruct every answer. |
| 127 | LeetCode 127: Word Ladder | Hard | Use breadth-first search to find the shortest transformation sequence length between two words. |
| 128 | LeetCode 128: Longest Consecutive Sequence | Medium | Find the longest run of consecutive integers in an unsorted array using a hash set and sequence-start detection. |
| 129 | LeetCode 129: Sum Root to Leaf Numbers | Medium | Compute the sum of all numbers formed by root-to-leaf paths using depth-first search and decimal accumulation. |
| 130 | LeetCode 130: Surrounded Regions | Medium | Capture surrounded O regions by marking border-connected O cells first, then flipping the remaining O cells. |
| 131 | LeetCode 131: Palindrome Partitioning | Medium | Generate all ways to split a string so that every piece is a palindrome, using backtracking with palindrome precomputation. |
| 132 | LeetCode 132: Palindrome Partitioning II | Hard | Find the minimum number of cuts needed to split a string into palindromic substrings using palindrome precomputation and dynamic programming. |
| 133 | LeetCode 133: Clone Graph | Medium | Create a deep copy of a connected undirected graph using DFS and a hash map from original nodes to cloned nodes. |
| 134 | LeetCode 134: Gas Station | Medium | Find the unique starting gas station index using a greedy scan with total fuel balance and current tank balance. |
| 135 | LeetCode 135: Candy | Hard | Compute the minimum candies needed using two greedy passes, one from the left and one from the right. |
| 136 | LeetCode 136: Single Number | Easy | Find the only number that appears once using the XOR operator, while every other number appears exactly twice. |
| 137 | LeetCode 137: Single Number II | Medium | Find the number that appears once when every other number appears three times using bit counting or finite-state bit manipulation. |
| 138 | LeetCode 138: Copy List with Random Pointer | Medium | Create a deep copy of a linked list with next and random pointers using hash maps or interleaved node cloning. |
| 139 | LeetCode 139: Word Break | Medium | Decide whether a string can be segmented into dictionary words using dynamic programming over prefixes. |
| 140 | LeetCode 140: Word Break II | Hard | Return all valid sentences formed by inserting spaces into a string so every word belongs to the dictionary, using DFS with memoization. |
| 141 | LeetCode 141: Linked List Cycle | Easy | Detect whether a linked list contains a cycle using Floyd’s tortoise and hare two-pointer algorithm. |
| 142 | LeetCode 142: Linked List Cycle II | Medium | Find the node where a linked list cycle begins using Floyd’s tortoise and hare algorithm with cycle entry mathematics. |
| 143 | LeetCode 143: Reorder List | Medium | Reorder a singly linked list in-place by finding the middle, reversing the second half, and merging the two halves alternately. |
| 144 | LeetCode 144: Binary Tree Preorder Traversal | Easy | Return the preorder traversal of a binary tree using recursion or an explicit stack. |
| 145 | LeetCode 145: Binary Tree Postorder Traversal | Easy | Return the postorder traversal of a binary tree using recursion or an iterative stack-based approach. |
| 146 | LeetCode 146: LRU Cache | Medium | Design an LRU cache with O(1) get and put operations using a hash map and doubly linked list. |
| 147 | LeetCode 147: Insertion Sort List | Medium | Sort a singly linked list using insertion sort by splicing each node into a growing sorted list. |
| 148 | LeetCode 148: Sort List | Medium | Sort a singly linked list in ascending order using merge sort with fast and slow pointers. |
| 149 | LeetCode 149: Max Points on a Line | Hard | Find the maximum number of points lying on the same straight line using slope counting and normalization. |
| 150 | LeetCode 150: Evaluate Reverse Polish Notation | Medium | Evaluate an arithmetic expression written in Reverse Polish Notation using a stack. |
| 151 | LeetCode 151: Reverse Words in a String | Medium | A clear explanation of reversing word order while removing extra spaces. |
| 152 | LeetCode 152: Maximum Product Subarray | Medium | A detailed explanation of tracking both maximum and minimum products while scanning the array. |
| 153 | LeetCode 153: Find Minimum in Rotated Sorted Array | Medium | A clear explanation of finding the minimum element in a rotated sorted array using binary search. |
| 154 | LeetCode 154: Find Minimum in Rotated Sorted Array II | Hard | A clear explanation of finding the minimum element in a rotated sorted array that may contain duplicates. |
| 155 | LeetCode 155: Min Stack | Medium | A clear explanation of designing a stack that can return the current minimum element in constant time. |
| 156 | LeetCode 156: Binary Tree Upside Down | Medium | A clear explanation of flipping a binary tree upside down by rewiring pointers from the left spine. |
| 157 | LeetCode 157: Read N Characters Given Read4 | Easy | A clear explanation of implementing read using the given read4 API and copying only the needed characters. |
| 158 | LeetCode 158: Read N Characters Given read4 II - Call Multiple Times | Hard | A clear explanation of implementing read with read4 when read may be called multiple times. |
| 159 | LeetCode 159: Longest Substring with At Most Two Distinct Characters | Medium | A clear explanation of finding the longest substring with at most two distinct characters using a sliding window. |
| 160 | LeetCode 160: Intersection of Two Linked Lists | Easy | A clear explanation of finding the node where two singly linked lists intersect using two pointers. |
| 161 | LeetCode 161: One Edit Distance | Medium | A clear explanation of checking whether two strings are exactly one edit apart using a linear scan. |
| 162 | LeetCode 162: Find Peak Element | Medium | A clear explanation of finding any peak element using binary search on the slope of the array. |
| 163 | LeetCode 163: Missing Ranges | Easy | A clear explanation of finding all missing ranges inside an inclusive interval by scanning sorted unique numbers. |
| 164 | LeetCode 164: Maximum Gap | Medium | A clear explanation of finding the maximum adjacent gap in sorted order using buckets and the pigeonhole principle. |
| 165 | LeetCode 165: Compare Version Numbers | Medium | A clear explanation of comparing version strings revision by revision while ignoring leading zeros. |
| 166 | LeetCode 166: Fraction to Recurring Decimal | Medium | A clear explanation of converting a fraction into decimal form and detecting repeating fractional parts with a hash map. |
| 167 | LeetCode 167: Two Sum II - Input Array Is Sorted | Medium | A clear explanation of finding two numbers in a sorted array using two pointers and constant extra space. |
| 168 | LeetCode 168: Excel Sheet Column Title | Easy | A clear explanation of converting a positive integer into an Excel column title using bijective base 26. |
| 169 | LeetCode 169: Majority Element | Easy | A clear explanation of finding the element that appears more than half the time using Boyer-Moore voting. |
| 170 | LeetCode 170: Two Sum III - Data Structure Design | Easy | A clear explanation of designing a data structure that supports add and find operations for pair sums. |
| 171 | LeetCode 171: Excel Sheet Column Number | Easy | A clear explanation of converting an Excel column title into its numeric index using base 26 accumulation. |
| 172 | LeetCode 172: Factorial Trailing Zeroes | Medium | A clear explanation of counting trailing zeroes in n! by counting factors of 5 instead of computing the factorial directly. |
| 173 | LeetCode 173: Binary Search Tree Iterator | Medium | A clear explanation of designing an iterator over a BST using controlled inorder traversal with a stack. |
| 174 | LeetCode 174: Dungeon Game | Hard | A clear explanation of computing the minimum initial health needed to survive a dungeon using reverse dynamic programming. |
| 175 | LeetCode 175: Combine Two Tables | Easy | A clear SQL guide for solving Combine Two Tables using LEFT JOIN. |
| 176 | LeetCode 176: Second Highest Salary | Medium | A clear SQL solution for finding the second highest distinct salary from the Employee table. |
| 177 | LeetCode 177: Nth Highest Salary | Medium | A clear SQL solution for finding the nth highest distinct salary from the Employee table. |
| 178 | LeetCode 178: Rank Scores | Medium | A clear SQL solution for ranking scores with dense ranking, where ties share the same rank and no rank numbers are skipped. |
| 179 | LeetCode 179: Largest Number | Medium | A clear explanation of arranging non-negative integers to form the largest possible concatenated number using a custom sort order. |
| 180 | LeetCode 180: Consecutive Numbers | Medium | A clear SQL solution for finding numbers that appear at least three times consecutively in the Logs table. |
| 181 | LeetCode 181: Employees Earning More Than Their Managers | Easy | A clear SQL solution for finding employees whose salary is greater than their manager's salary using a self join. |
| 182 | LeetCode 182: Duplicate Emails | Easy | A clear SQL solution for reporting email values that appear more than once in the Person table. |
| 183 | LeetCode 183: Customers Who Never Order | Easy | A clear SQL solution for finding customers who have no matching rows in the Orders table. |
| 184 | LeetCode 184: Department Highest Salary | Medium | A clear SQL solution for finding every employee who earns the highest salary in their department. |
| 185 | LeetCode 185: Department Top Three Salaries | Hard | A clear SQL solution for finding employees whose salaries are in the top three unique salary levels within their department. |
| 186 | LeetCode 186: Reverse Words in a String II | Medium | A clear explanation of reversing the order of words in a character array in-place using two reversals. |
| 187 | LeetCode 187: Repeated DNA Sequences | Medium | A clear explanation of finding repeated 10-letter DNA substrings using a fixed-size sliding window and hash sets. |
| 188 | LeetCode 188: Best Time to Buy and Sell Stock IV | Hard | A clear explanation of maximizing stock trading profit with at most k transactions using dynamic programming. |
| 189 | LeetCode 189: Rotate Array | Medium | A clear explanation of rotating an array to the right by k steps using in-place reversal. |
| 190 | LeetCode 190: Reverse Bits | Easy | A clear explanation of reversing the bits of a 32-bit integer using bit manipulation. |
| 191 | LeetCode 191: Number of 1 Bits | Easy | A clear explanation of counting set bits in an integer using bit manipulation and Brian Kernighan's algorithm. |
| 192 | LeetCode 192: Word Frequency | Medium | A clear explanation of the Word Frequency shell problem using Unix text-processing tools. |
| 193 | LeetCode 193: Valid Phone Numbers | Easy | A clear explanation of the Valid Phone Numbers shell problem using grep and regular expressions. |
| 194 | LeetCode 194: Transpose File | Medium | A clear explanation of the Transpose File shell problem using awk to transform rows into columns. |
| 195 | LeetCode 195: Tenth Line | Easy | A clear explanation of the Tenth Line shell problem using awk, sed, head, and tail. |
| 196 | LeetCode 196: Delete Duplicate Emails | Easy | A clear explanation of the Delete Duplicate Emails SQL problem using DELETE with a self join. |
| 197 | LeetCode 197: Rising Temperature | Easy | A clear explanation of the Rising Temperature SQL problem using a self join and date comparison. |
| 198 | LeetCode 198: House Robber | Medium | A clear explanation of maximizing robbery profit without robbing adjacent houses using dynamic programming. |
| 199 | LeetCode 199: Binary Tree Right Side View | Medium | A clear explanation of returning the visible nodes from the right side of a binary tree using level-order traversal. |