LeetCode 04xx
LeetCode practice notes for problems 400 through 499, including A clear explanation of finding the nth digit in the infinite integer sequence using digit groups and arithmetic.
LeetCode 04xx
| # | Title | Difficulty | Description |
|---|---|---|---|
| 400 | LeetCode 400: Nth Digit | Medium | A clear explanation of finding the nth digit in the infinite integer sequence using digit groups and arithmetic. |
| 401 | LeetCode 401: Binary Watch | Easy | A clear explanation of the Binary Watch problem using bit counting over all valid times. |
| 402 | LeetCode 402: Remove K Digits | Medium | A clear explanation of the Remove K Digits problem using a greedy monotonic stack. |
| 403 | LeetCode 403: Frog Jump | Hard | A clear explanation of the Frog Jump problem using dynamic programming with reachable jump sizes. |
| 404 | LeetCode 404: Sum of Left Leaves | Easy | A clear explanation of the Sum of Left Leaves problem using depth-first traversal of a binary tree. |
| 405 | LeetCode 405: Convert a Number to Hexadecimal | Easy | A clear explanation of converting integers to hexadecimal using bit manipulation and two's complement representation. |
| 406 | LeetCode 406: Queue Reconstruction by Height | Medium | A clear explanation of reconstructing a queue using greedy sorting and indexed insertion. |
| 407 | LeetCode 407: Trapping Rain Water II | Hard | A clear explanation of trapping rain water in a 2D elevation map using a min heap and boundary expansion. |
| 408 | LeetCode 408: Valid Word Abbreviation | Easy | A clear explanation of validating a word abbreviation using two pointers and number parsing. |
| 409 | LeetCode 409: Longest Palindrome | Easy | A clear explanation of finding the longest palindrome length that can be built from given letters using character counts. |
| 410 | LeetCode 410: Split Array Largest Sum | Hard | A clear explanation of minimizing the largest subarray sum using binary search on the answer and greedy validation. |
| 411 | LeetCode 411: Minimum Unique Word Abbreviation | Hard | A clear explanation of finding the shortest abbreviation that does not conflict with any dictionary word using bit masks. |
| 412 | LeetCode 412: Fizz Buzz | Easy | A clear explanation of the Fizz Buzz problem using direct simulation and divisibility checks. |
| 413 | LeetCode 413: Arithmetic Slices | Medium | A clear explanation of counting arithmetic subarrays using dynamic programming and consecutive differences. |
| 414 | LeetCode 414: Third Maximum Number | Easy | A clear explanation of finding the third distinct maximum number using one pass and constant space. |
| 415 | LeetCode 415: Add Strings | Easy | A clear explanation of adding two non-negative integer strings using manual digit-by-digit simulation. |
| 416 | LeetCode 416: Partition Equal Subset Sum | Medium | A clear explanation of deciding whether an array can be split into two equal-sum subsets using 0/1 knapsack dynamic programming. |
| 417 | LeetCode 417: Pacific Atlantic Water Flow | Medium | A clear explanation of finding cells that can flow to both oceans using reverse graph traversal from the borders. |
| 418 | LeetCode 418: Sentence Screen Fitting | Medium | A clear explanation of fitting a sentence onto a screen using cyclic string simulation and greedy row transitions. |
| 419 | LeetCode 419: Battleships in a Board | Medium | A clear explanation of counting battleships in a board using one-pass observation without modifying the grid. |
| 420 | LeetCode 420: Strong Password Checker | Hard | A clear explanation of checking the minimum edits needed to make a password strong using greedy handling of length, missing character types, and repeated runs. |
| 421 | LeetCode 421: Maximum XOR of Two Numbers in an Array | Medium | A clear explanation of finding the maximum XOR of two numbers using greedy bit prefixes. |
| 422 | LeetCode 422: Valid Word Square | Easy | A clear explanation of checking whether rows and columns read the same using direct index comparison. |
| 423 | LeetCode 423: Reconstruct Original Digits from English | Medium | A clear explanation of reconstructing digits from shuffled English words using character frequency counts and unique identifying letters. |
| 424 | LeetCode 424: Longest Repeating Character Replacement | Medium | A clear explanation of finding the longest substring that can become all one letter using a sliding window. |
| 425 | LeetCode 425: Word Squares | Hard | A clear explanation of building all word squares using backtracking with prefix pruning. |
| 426 | LeetCode 426: Convert Binary Search Tree to Sorted Doubly Linked List | Medium | Convert a BST into a sorted circular doubly linked list in-place using inorder traversal. |
| 427 | LeetCode 427: Construct Quad Tree | Medium | Build a quad tree from a binary square grid using recursive divide and conquer. |
| 428 | LeetCode 428: Serialize and Deserialize N-ary Tree | Hard | Serialize an N-ary tree into a string and reconstruct the same tree using preorder traversal with child counts. |
| 429 | LeetCode 429: N-ary Tree Level Order Traversal | Medium | Traverse an N-ary tree level by level using breadth-first search. |
| 430 | LeetCode 430: Flatten a Multilevel Doubly Linked List | Medium | Flatten a multilevel doubly linked list in-place using depth-first traversal and pointer splicing. |
| 431 | LeetCode 431: Encode N-ary Tree to Binary Tree | Hard | Convert an N-ary tree into a binary tree and reconstruct it using the left-child right-sibling representation. |
| 432 | LeetCode 432: All O'one Data Structure | Hard | Design a data structure that supports increment, decrement, get minimum key, and get maximum key in average O(1) time. |
| 433 | LeetCode 433: Minimum Genetic Mutation | Medium | Find the minimum number of valid one-character gene mutations using breadth-first search. |
| 434 | LeetCode 434: Number of Segments in a String | Easy | Count the number of word segments in a string by detecting transitions from spaces to non-space characters. |
| 435 | LeetCode 435: Non-overlapping Intervals | Medium | Remove the minimum number of intervals so the remaining intervals do not overlap, using greedy sorting by end time. |
| 436 | LeetCode 436: Find Right Interval | Medium | Find, for each interval, the interval with the smallest start point greater than or equal to its end point using sorting and binary search. |
| 437 | LeetCode 437: Path Sum III | Medium | Count downward paths in a binary tree whose values sum to targetSum using DFS and prefix sums. |
| 438 | LeetCode 438: Find All Anagrams in a String | Medium | Find all starting indices where an anagram of p appears in s using a fixed-size sliding window. |
| 439 | LeetCode 439: Ternary Expression Parser | Medium | Evaluate a nested ternary expression using a right-to-left stack parser. |
| 440 | LeetCode 440: K-th Smallest in Lexicographical Order | Hard | Find the k-th integer in lexicographical order without generating all numbers, using prefix counting over a conceptual trie. |
| 441 | LeetCode 441: Arranging Coins | Easy | Find the maximum number of complete staircase rows that can be formed using binary search and triangular numbers. |
| 442 | LeetCode 442: Find All Duplicates in an Array | Medium | Find all duplicated numbers in an array in O(n) time and O(1) extra space using index marking. |
| 443 | LeetCode 443: String Compression | Medium | Compress a character array in-place using two pointers and grouped character counting. |
| 444 | LeetCode 444: Sequence Reconstruction | Medium | Check whether nums is the unique shortest supersequence of given subsequences using topological sorting. |
| 445 | LeetCode 445: Add Two Numbers II | Medium | Add two numbers stored in forward-order linked lists using stacks and carry propagation. |
| 446 | LeetCode 446: Arithmetic Slices II - Subsequence | Hard | Count arithmetic subsequences of length at least three using dynamic programming with one hash map per ending index. |
| 447 | LeetCode 447: Number of Boomerangs | Medium | Count ordered boomerang tuples by fixing each point as the center and grouping other points by squared distance. |
| 448 | LeetCode 448: Find All Numbers Disappeared in an Array | Easy | Find all missing numbers from 1 to n in O(n) time using in-place index marking. |
| 449 | LeetCode 449: Serialize and Deserialize BST | Medium | Serialize a binary search tree compactly with preorder traversal and rebuild it using BST value bounds. |
| 450 | LeetCode 450: Delete Node in a BST | Medium | Delete a node from a binary search tree while preserving the BST property using recursive search and inorder successor replacement. |
| 451 | LeetCode 451: Sort Characters By Frequency | Medium | A clear explanation of sorting characters by decreasing frequency using a hash map and sorting. |
| 452 | LeetCode 452: Minimum Number of Arrows to Burst Balloons | Medium | A clear explanation of the greedy interval solution for finding the minimum number of arrows needed to burst all balloons. |
| 453 | LeetCode 453: Minimum Moves to Equal Array Elements | Medium | A clear explanation of the math behind making all array elements equal by incrementing n - 1 elements at a time. |
| 454 | LeetCode 454: 4Sum II | Medium | A clear explanation of counting zero-sum tuples across four arrays using pair sums and a hash map. |
| 455 | LeetCode 455: Assign Cookies | Easy | A clear explanation of the greedy two-pointer solution for maximizing the number of content children. |
| 456 | LeetCode 456: 132 Pattern | Medium | A clear explanation of detecting a 132 pattern using reverse traversal and a monotonic stack. |
| 457 | LeetCode 457: Circular Array Loop | Medium | A clear explanation of detecting a valid cycle in a circular array using fast and slow pointers. |
| 458 | LeetCode 458: Poor Pigs | Hard | A clear explanation of the combinatorics behind finding the minimum number of pigs needed to identify the poisonous bucket. |
| 459 | LeetCode 459: Repeated Substring Pattern | Easy | A clear explanation of checking whether a string can be built by repeating one of its proper substrings. |
| 460 | LeetCode 460: LFU Cache | Hard | A clear explanation of designing an LFU cache with O(1) average get and put operations. |
| 461 | LeetCode 461: Hamming Distance | Easy | A clear explanation of computing the Hamming distance between two integers using XOR and bit counting. |
| 462 | LeetCode 462: Minimum Moves to Equal Array Elements II | Medium | A clear explanation of why the median minimizes the number of moves needed to make all array elements equal. |
| 463 | LeetCode 463: Island Perimeter | Easy | A clear explanation of counting the perimeter of an island in a grid by adding land-cell edges and subtracting shared edges. |
| 464 | LeetCode 464: Can I Win | Medium | A clear explanation of solving the Can I Win game using minimax recursion, bitmask state compression, and memoization. |
| 465 | LeetCode 465: Optimal Account Balancing | Hard | A clear explanation of minimizing debt-settlement transactions using net balances, backtracking, and memoization-style pruning. |
| 466 | LeetCode 466: Count The Repetitions | Hard | A clear explanation of counting how many repeated copies of one string can be obtained as a subsequence of another repeated string. |
| 467 | LeetCode 467: Unique Substrings in Wraparound String | Medium | A clear explanation of counting unique substrings that appear in the infinite alphabet wraparound string using dynamic programming by ending character. |
| 468 | LeetCode 468: Validate IP Address | Medium | A clear explanation of validating IPv4 and IPv6 addresses by checking segment count, length, characters, range, and leading-zero rules. |
| 469 | LeetCode 469: Convex Polygon | Medium | A clear explanation of checking whether ordered points form a convex polygon using cross products. |
| 470 | LeetCode 470: Implement Rand10() Using Rand7() | Medium | A clear explanation of generating a uniform random integer from 1 to 10 using only rand7 and rejection sampling. |
| 471 | LeetCode 471: Encode String with Shortest Length | Hard | A clear explanation of interval dynamic programming for encoding a string into the shortest k[encoded_string] form. |
| 472 | LeetCode 472: Concatenated Words | Hard | A clear explanation of finding all words that can be formed by concatenating at least two shorter words from the same list. |
| 473 | LeetCode 473: Matchsticks to Square | Medium | A clear explanation of deciding whether matchsticks can form a square using backtracking, sorting, and pruning. |
| 474 | LeetCode 474: Ones and Zeroes | Medium | A clear explanation of solving the largest subset problem as a two-dimensional 0/1 knapsack over zero and one counts. |
| 475 | LeetCode 475: Heaters | Medium | A clear explanation of finding the minimum heater radius by sorting positions and matching each house to its nearest heater. |
| 476 | LeetCode 476: Number Complement | Easy | A clear explanation of finding the bitwise complement of a positive integer using a binary mask. |
| 477 | LeetCode 477: Total Hamming Distance | Medium | A clear explanation of computing the total Hamming distance across all pairs by counting different bits column by column. |
| 478 | LeetCode 478: Generate Random Point in a Circle | Medium | A clear explanation of generating uniformly random points inside a circle using polar coordinates. |
| 479 | LeetCode 479: Largest Palindrome Product | Hard | A clear explanation of finding the largest palindrome made from the product of two n-digit numbers by generating palindrome candidates directly. |
| 480 | LeetCode 480: Sliding Window Median | Hard | A clear explanation of maintaining the median of each fixed-size window using two heaps and lazy deletion. |
| 481 | LeetCode 481: Magical String | Medium | A clear explanation of constructing the magical string by using the string itself as run-length instructions. |
| 482 | LeetCode 482: License Key Formatting | Easy | A clear explanation of reformatting a license key by removing dashes, uppercasing characters, and grouping from the right. |
| 483 | LeetCode 483: Smallest Good Base | Hard | A clear explanation of finding the smallest base where n is written as all ones using geometric series and binary search. |
| 484 | LeetCode 484: Find Permutation | Medium | A clear explanation of constructing the lexicographically smallest permutation that matches an I and D pattern. |
| 485 | LeetCode 485: Max Consecutive Ones | Easy | A clear explanation of finding the longest streak of 1s in a binary array with a single pass. |
| 486 | LeetCode 486: Predict the Winner | Medium | A clear explanation of predicting whether Player 1 can win using minimax dynamic programming over score difference. |
| 487 | LeetCode 487: Max Consecutive Ones II | Medium | A clear explanation of finding the longest run of 1s after flipping at most one 0 using a sliding window. |
| 488 | LeetCode 488: Zuma Game | Hard | A clear explanation of solving Zuma Game with DFS, memoization, and chain-removal simulation. |
| 489 | LeetCode 489: Robot Room Cleaner | Hard | A clear explanation of cleaning an unknown grid using DFS, relative coordinates, and physical backtracking. |
| 490 | LeetCode 490: The Maze | Medium | A clear explanation of deciding whether a rolling ball can stop at the destination using BFS or DFS over stopping cells. |
| 491 | LeetCode 491: Non-decreasing Subsequences | Medium | A clear explanation of generating all distinct non-decreasing subsequences using DFS, backtracking, and per-level duplicate control. |
| 492 | LeetCode 492: Construct the Rectangle | Easy | A clear explanation of finding rectangle dimensions with a fixed area and the smallest length-width difference. |
| 493 | LeetCode 493: Reverse Pairs | Hard | A clear explanation of counting pairs where nums[i] is greater than twice nums[j] using merge sort. |
| 494 | LeetCode 494: Target Sum | Medium | A clear explanation of counting sign assignments that reach a target using recursion first, then subset-sum dynamic programming. |
| 495 | LeetCode 495: Teemo Attacking | Easy | A clear explanation of calculating total poisoned duration by merging overlapping attack intervals. |
| 496 | LeetCode 496: Next Greater Element I | Easy | A clear explanation of finding the next greater element using a monotonic decreasing stack and hash map. |
| 497 | LeetCode 497: Random Point in Non-overlapping Rectangles | Medium | A clear explanation of uniformly picking an integer point from non-overlapping rectangles using prefix sums and binary search. |
| 498 | LeetCode 498: Diagonal Traverse | Medium | A clear explanation of returning matrix elements in diagonal zigzag order by grouping cells with the same row plus column index. |
| 499 | LeetCode 499: The Maze III | Hard | A clear explanation of finding the shortest rolling-ball path to the hole using Dijkstra with lexicographic tie-breaking. |