LeetCode 09xx
LeetCode practice notes for problems 900 through 999, including A clear explanation of designing an iterator over a run-length encoded sequence without expanding it.
LeetCode 09xx
| # | Title | Difficulty | Description |
|---|---|---|---|
| 900 | LeetCode 900: RLE Iterator | Medium | A clear explanation of designing an iterator over a run-length encoded sequence without expanding it. |
| 901 | LeetCode 901: Online Stock Span | Medium | A clear explanation of Online Stock Span using a monotonic decreasing stack with accumulated spans. |
| 902 | LeetCode 902: Numbers At Most N Given Digit Set | Hard | A clear explanation of counting numbers less than or equal to N using digit-by-digit construction and combinatorics. |
| 903 | LeetCode 903: Valid Permutations for DI Sequence | Hard | A clear explanation of counting valid DI permutations using dynamic programming and prefix sums. |
| 904 | LeetCode 904: Fruit Into Baskets | Medium | A clear explanation of Fruit Into Baskets using a sliding window with at most two distinct fruit types. |
| 905 | LeetCode 905: Sort Array By Parity | Easy | A clear explanation of sorting an array by parity using a two-pointer partition method. |
| 906 | LeetCode 906: Super Palindromes | Hard | A clear explanation of counting super-palindromes by generating palindromic roots and checking their squares. |
| 907 | LeetCode 907: Sum of Subarray Minimums | Medium | A clear explanation of summing subarray minimums using a monotonic stack and contribution counting. |
| 908 | LeetCode 908: Smallest Range I | Easy | A clear explanation of minimizing an array score after each value can move by at most k. |
| 909 | LeetCode 909: Snakes and Ladders | Medium | A clear explanation of Snakes and Ladders using breadth-first search over board squares. |
| 910 | LeetCode 910: Smallest Range II | Medium | A clear explanation of minimizing the array range after adding either +k or -k to every element. |
| 911 | LeetCode 911: Online Election | Medium | A clear explanation of Online Election using preprocessing and binary search over vote times. |
| 912 | LeetCode 912: Sort an Array | Medium | A clear explanation of sorting an array without built-in sorting using merge sort. |
| 913 | LeetCode 913: Cat and Mouse | Hard | A clear explanation of Cat and Mouse using game states, reverse BFS, and topological propagation. |
| 914 | LeetCode 914: X of a Kind in a Deck of Cards | Easy | A clear explanation of checking whether card counts share a common group size using the greatest common divisor. |
| 915 | LeetCode 915: Partition Array into Disjoint Intervals | Medium | A clear explanation of finding the smallest left partition using prefix maximums and suffix minimums. |
| 916 | LeetCode 916: Word Subsets | Medium | A clear explanation of finding universal words by merging character frequency requirements from words2. |
| 917 | LeetCode 917: Reverse Only Letters | Easy | A clear explanation of reversing only English letters while keeping all non-letter characters fixed. |
| 918 | LeetCode 918: Maximum Sum Circular Subarray | Medium | A clear explanation of finding the maximum circular subarray sum using Kadane's algorithm. |
| 919 | LeetCode 919: Complete Binary Tree Inserter | Medium | A clear explanation of maintaining a complete binary tree inserter using level-order indexing. |
| 920 | LeetCode 920: Number of Music Playlists | Hard | A clear explanation of counting valid music playlists using dynamic programming over playlist length and unique songs used. |
| 921 | LeetCode 921: Minimum Add to Make Parentheses Valid | Medium | A clear explanation of making a parentheses string valid using greedy counting. |
| 922 | LeetCode 922: Sort Array By Parity II | Easy | A clear explanation of placing even numbers at even indices and odd numbers at odd indices using two pointers. |
| 923 | LeetCode 923: 3Sum With Multiplicity | Medium | A clear explanation of counting index triplets with duplicate values using frequency counts and combinatorics. |
| 924 | LeetCode 924: Minimize Malware Spread | Hard | A clear explanation of minimizing malware spread by analyzing connected components with Union Find. |
| 925 | LeetCode 925: Long Pressed Name | Easy | A clear explanation of the Long Pressed Name problem using a two-pointer scan. |
| 926 | LeetCode 926: Flip String to Monotone Increasing | Medium | A clear explanation of solving Flip String to Monotone Increasing with a one-pass dynamic programming approach. |
| 927 | LeetCode 927: Three Equal Parts | Hard | A clear explanation of solving Three Equal Parts by counting ones, locating the three binary patterns, and comparing them in one pass. |
| 928 | LeetCode 928: Minimize Malware Spread II | Hard | A clear explanation of solving Minimize Malware Spread II by removing each infected node and simulating the final malware spread. |
| 929 | LeetCode 929: Unique Email Addresses | Easy | A clear explanation of solving Unique Email Addresses using string normalization and a hash set. |
| 930 | LeetCode 930: Binary Subarrays With Sum | Medium | A clear explanation of solving Binary Subarrays With Sum using prefix sums and a frequency map. |
| 931 | LeetCode 931: Minimum Falling Path Sum | Medium | A clear explanation of solving Minimum Falling Path Sum using dynamic programming over matrix rows. |
| 932 | LeetCode 932: Beautiful Array | Medium | A clear explanation of solving Beautiful Array using divide and conquer with odd and even transformations. |
| 933 | LeetCode 933: Number of Recent Calls | Easy | A clear explanation of solving Number of Recent Calls using a queue as a sliding time window. |
| 934 | LeetCode 934: Shortest Bridge | Medium | A clear explanation of solving Shortest Bridge using DFS to mark one island and BFS to expand toward the other island. |
| 935 | LeetCode 935: Knight Dialer | Medium | A clear explanation of solving Knight Dialer using dynamic programming over the phone keypad graph. |
| 936 | LeetCode 936: Stamping The Sequence | Hard | A clear explanation of solving Stamping The Sequence using reverse simulation and BFS-style processing. |
| 937 | LeetCode 937: Reorder Data in Log Files | Easy | A clear explanation of solving Reorder Data in Log Files using custom sorting and stable handling of digit logs. |
| 938 | LeetCode 938: Range Sum of BST | Easy | A clear explanation of solving Range Sum of BST using DFS with binary search tree pruning. |
| 939 | LeetCode 939: Minimum Area Rectangle | Medium | A clear explanation of solving Minimum Area Rectangle using diagonal point pairs and constant-time point lookup. |
| 940 | LeetCode 940: Distinct Subsequences II | Hard | A clear explanation of solving Distinct Subsequences II using dynamic programming and last occurrence tracking. |
| 941 | LeetCode 941: Valid Mountain Array | Easy | A clear explanation of solving Valid Mountain Array by walking up the increasing slope and then down the decreasing slope. |
| 942 | LeetCode 942: DI String Match | Easy | A clear explanation of solving DI String Match using a greedy two-pointer construction. |
| 943 | LeetCode 943: Find the Shortest Superstring | Hard | A clear explanation of solving Find the Shortest Superstring using pairwise overlaps and bitmask dynamic programming. |
| 944 | LeetCode 944: Delete Columns to Make Sorted | Easy | A clear explanation of solving Delete Columns to Make Sorted by checking each column independently. |
| 945 | LeetCode 945: Minimum Increment to Make Array Unique | Medium | A clear explanation of solving Minimum Increment to Make Array Unique by sorting and greedily assigning the next available value. |
| 946 | LeetCode 946: Validate Stack Sequences | Medium | A clear explanation of solving Validate Stack Sequences by simulating stack push and pop operations. |
| 947 | LeetCode 947: Most Stones Removed with Same Row or Column | Medium | A clear explanation of solving Most Stones Removed with Same Row or Column using connected components and union-find. |
| 948 | LeetCode 948: Bag of Tokens | Medium | A clear explanation of solving Bag of Tokens using sorting, greedy choices, and two pointers. |
| 949 | LeetCode 949: Largest Time for Given Digits | Medium | A clear explanation of solving Largest Time for Given Digits by checking all permutations of four digits. |
| 950 | LeetCode 950: Reveal Cards In Increasing Order | Medium | A clear explanation of solving Reveal Cards In Increasing Order using sorting and queue simulation over indices. |
| 951 | LeetCode 951: Flip Equivalent Binary Trees | Medium | A clear explanation of checking whether two binary trees are equivalent after swapping left and right children at any number of nodes. |
| 952 | LeetCode 952: Largest Component Size by Common Factor | Hard | A clear explanation of solving Largest Component Size by Common Factor using prime factorization and union find. |
| 953 | LeetCode 953: Verifying an Alien Dictionary | Easy | A clear explanation of checking whether words are sorted according to a custom alien alphabet order. |
| 954 | LeetCode 954: Array of Doubled Pairs | Medium | A clear explanation of checking whether an array can be reordered into pairs where one number is double the other. |
| 955 | LeetCode 955: Delete Columns to Make Sorted II | Medium | A clear explanation of deleting the minimum number of columns so rows become lexicographically sorted. |
| 956 | LeetCode 956: Tallest Billboard | Hard | A clear explanation of solving Tallest Billboard using dynamic programming over height differences. |
| 957 | LeetCode 957: Prison Cells After N Days | Medium | A clear explanation of simulating prison cell transitions efficiently using cycle detection. |
| 958 | LeetCode 958: Check Completeness of a Binary Tree | Medium | A clear explanation of checking whether a binary tree is complete using level-order traversal. |
| 959 | LeetCode 959: Regions Cut By Slashes | Medium | A clear explanation of counting regions formed by slashes using union find over four triangles per cell. |
| 960 | LeetCode 960: Delete Columns to Make Sorted III | Hard | A clear explanation of deleting the minimum number of columns so every remaining row is individually sorted. |
| 961 | LeetCode 961: N-Repeated Element in Size 2N Array | Easy | A clear explanation of finding the element repeated N times using a hash set. |
| 962 | LeetCode 962: Maximum Width Ramp | Medium | A clear explanation of finding the maximum width ramp using a monotonic decreasing stack. |
| 963 | LeetCode 963: Minimum Area Rectangle II | Medium | A clear explanation of finding the minimum-area rectangle from points when the rectangle may be rotated. |
| 964 | LeetCode 964: Least Operators to Express Number | Hard | A clear explanation of expressing a target using the fewest operators with repeated uses of x. |
| 965 | LeetCode 965: Univalued Binary Tree | Easy | A clear explanation of checking whether every node in a binary tree has the same value. |
| 966 | LeetCode 966: Vowel Spellchecker | Medium | A clear explanation of implementing a spellchecker with exact, case-insensitive, and vowel-error matching. |
| 967 | LeetCode 967: Numbers With Same Consecutive Differences | Medium | A clear explanation of generating all n-digit numbers whose adjacent digits differ by k. |
| 968 | LeetCode 968: Binary Tree Cameras | Hard | A clear explanation of placing the minimum number of cameras in a binary tree using postorder DFS. |
| 969 | LeetCode 969: Pancake Sorting | Medium | A clear explanation of sorting an array using prefix reversals by repeatedly placing the largest remaining value. |
| 970 | LeetCode 970: Powerful Integers | Medium | A clear explanation of generating all powerful integers using bounded powers and a set. |
| 971 | LeetCode 971: Flip Binary Tree To Match Preorder Traversal | Medium | A clear explanation of matching a binary tree preorder traversal by greedily flipping nodes. |
| 972 | LeetCode 972: Equal Rational Numbers | Hard | A clear explanation of comparing rational numbers written as decimal strings with optional repeating parts. |
| 973 | LeetCode 973: K Closest Points to Origin | Medium | A clear explanation of returning the k closest points to the origin using squared distance and sorting. |
| 974 | LeetCode 974: Subarray Sums Divisible by K | Medium | A clear explanation of counting subarrays whose sum is divisible by k using prefix sums and remainder frequencies. |
| 975 | LeetCode 975: Odd Even Jump | Hard | A clear explanation of counting good starting indices using next-jump preprocessing and dynamic programming. |
| 976 | LeetCode 976: Largest Perimeter Triangle | Easy | A clear explanation of finding the largest valid triangle perimeter using sorting and a greedy scan. |
| 977 | LeetCode 977: Squares of a Sorted Array | Easy | A clear explanation of sorting squared values from a sorted array using two pointers. |
| 978 | LeetCode 978: Longest Turbulent Subarray | Medium | A clear explanation of finding the longest subarray whose adjacent comparisons alternate between greater-than and less-than. |
| 979 | LeetCode 979: Distribute Coins in Binary Tree | Medium | A clear explanation of balancing coins in a binary tree using postorder DFS and subtree coin balance. |
| 980 | LeetCode 980: Unique Paths III | Hard | A clear explanation of counting all paths from start to end that visit every non-obstacle square exactly once using backtracking. |
| 981 | LeetCode 981: Time Based Key-Value Store | Medium | A clear explanation of designing a time-based key-value store using a hash map and binary search. |
| 982 | LeetCode 982: Triples with Bitwise AND Equal To Zero | Hard | A clear explanation of counting ordered triples whose bitwise AND is zero using pairwise AND counts. |
| 983 | LeetCode 983: Minimum Cost For Tickets | Medium | A clear explanation of finding the cheapest way to cover all travel days using dynamic programming. |
| 984 | LeetCode 984: String Without AAA or BBB | Medium | A clear explanation of constructing a string with exact counts of a and b while avoiding three equal consecutive characters. |
| 985 | LeetCode 985: Sum of Even Numbers After Queries | Medium | A clear explanation of maintaining the sum of even numbers after each array update. |
| 986 | LeetCode 986: Interval List Intersections | Medium | A clear explanation of finding intersections between two sorted disjoint interval lists using two pointers. |
| 987 | LeetCode 987: Vertical Order Traversal of a Binary Tree | Hard | A clear explanation of vertical tree traversal using coordinates, DFS, sorting, and column grouping. |
| 988 | LeetCode 988: Smallest String Starting From Leaf | Medium | A clear explanation of finding the lexicographically smallest leaf-to-root string in a binary tree using DFS. |
| 989 | LeetCode 989: Add to Array-Form of Integer | Easy | A clear explanation of adding an integer to an array-form number using digit-by-digit simulation. |
| 990 | LeetCode 990: Satisfiability of Equality Equations | Medium | A clear explanation of checking equality and inequality constraints using union-find. |
| 991 | LeetCode 991: Broken Calculator | Medium | A clear explanation of finding the minimum operations by working backward from target to startValue. |
| 992 | LeetCode 992: Subarrays with K Different Integers | Hard | A clear explanation of counting subarrays with exactly k distinct integers using the at-most-k sliding window trick. |
| 993 | LeetCode 993: Cousins in Binary Tree | Easy | A clear explanation of checking whether two binary tree nodes are cousins using BFS with parent tracking. |
| 994 | LeetCode 994: Rotting Oranges | Medium | A clear explanation of finding the minimum time for all oranges to rot using multi-source BFS. |
| 995 | LeetCode 995: Minimum Number of K Consecutive Bit Flips | Hard | A clear explanation of making all bits equal to 1 using greedy left-to-right flips and a sliding window flip parity. |
| 996 | LeetCode 996: Number of Squareful Arrays | Hard | A clear explanation of counting unique permutations where every adjacent pair sums to a perfect square using backtracking. |
| 997 | LeetCode 997: Find the Town Judge | Easy | A clear explanation of identifying the town judge using trust indegree and outdegree counts. |
| 998 | LeetCode 998: Maximum Binary Tree II | Medium | A clear explanation of inserting a value into a maximum binary tree by following the right spine. |
| 999 | LeetCode 999: Available Captures for Rook | Easy | A clear explanation of counting how many pawns a rook can capture by scanning four directions on a chessboard. |