Coding interview solutions

Two Sum solution

Find the indices of two distinct values whose sum matches a target.

Merge Intervals solution

Sort and merge every overlapping or touching interval into a disjoint list.

Reverse a String solution

Return a string with every character in reverse order without changing characters.

Longest Common Prefix solution

Find the longest leading substring shared by every string in a list.

Palindrome String solution

Determine whether a phrase reads the same after ignoring case and non-alphanumeric characters.

Palindromic Substrings solution

Count every contiguous substring that reads the same forward and backward.

Longest Palindromic Substring solution

Return the longest contiguous substring that is a palindrome.

Palindrome Linked List solution

Check whether the values represented by a singly linked list form a palindrome.

Roman to Integer solution

Convert a valid canonical Roman numeral into its integer value.

Letter Combinations of a Phone Number solution

Generate every letter combination represented by digits on a telephone keypad.

Valid Parentheses solution

Validate that every bracket closes in the correct order and with the matching type.

Find the Index of the First Occurrence in a String solution

Return the starting index of the first exact occurrence of one string inside another.

Length of Last Word solution

Measure the final whitespace-delimited word in a string.

Spiral Matrix solution

Generate an n-by-n matrix filled clockwise in spiral order.

FizzBuzz solution

Produce the classic FizzBuzz sequence using divisibility rules.

Contains Duplicate solution

Determine whether any integer appears more than once in an array.

Maximum Subarray solution

Return the largest sum obtainable from a non-empty contiguous subarray.

Fibonacci Number solution

Compute the zero-indexed Fibonacci number with an iterative recurrence.

Factorial solution

Compute the product of all positive integers up to a non-negative input.

Move Zeroes solution

Move every zero to the end while preserving non-zero element order.

Single Number solution

Find the only value that is not paired using constant extra space.

Best Time to Buy and Sell Stock solution

Find the best profit from one buy followed by one later sale.

Valid Anagram solution

Check whether two lowercase strings contain identical character frequencies.

Binary Search solution

Locate a target in a sorted unique array using logarithmic search.

Count Vowels solution

Count ASCII vowels in a string without changing other characters.

Climbing Stairs solution

Count ordered one-step and two-step paths to the top of a staircase.

Product of Array Except Self solution

Build prefix and suffix products without division.

Trapping Rain Water solution

Compute water trapped between elevation bars after rainfall.

Edit Distance solution

Measure the minimum insertions, deletions, and replacements between two strings.

Median of Two Sorted Arrays solution

Find the combined median without fully merging two sorted arrays.

Longest Valid Parentheses solution

Measure the longest contiguous substring of balanced parentheses.

Palindrome Number solution

Determine whether a base-10 integer reads identically in reverse.

Plus One solution

Add one to a number represented as an array of digits.

Majority Element solution

Find the element that appears more than half the time in an array.

Is Subsequence solution

Check whether one string is a subsequence of another.

First Unique Character in a String solution

Find the index of the first non-repeating character in a string.

Search Insert Position solution

Return the index where a target should be inserted into a sorted array.

House Robber solution

Maximize the loot from non-adjacent houses along a street.

Jump Game solution

Decide whether you can reach the last index of an array of jump lengths.

Coin Change solution

Find the fewest coins needed to make up a given amount.

Longest Substring Without Repeating Characters solution

Find the length of the longest substring without repeating characters.

Container With Most Water solution

Find two lines that together with the x-axis hold the most water.

Number of Islands solution

Count the number of islands in a grid of land and water cells.

Word Break solution

Determine whether a string can be segmented into words from a dictionary.

Longest Increasing Subsequence solution

Find the length of the longest strictly increasing subsequence.

Largest Rectangle in Histogram solution

Find the area of the largest rectangle that fits under a histogram.

Sliding Window Maximum solution

Return the maximum of every contiguous window of size k.

Jump Game II solution

Find the minimum number of jumps needed to reach the last index.

Reverse Linked List solution

Reverse a singly linked list in place using real ListNode structures.

Maximum Depth of Binary Tree solution

Find the maximum depth of a binary tree using real TreeNode structures.

Course Schedule solution

Determine whether all courses can be completed given prerequisite pairs.

Rotting Oranges solution

Find how many minutes are needed for rot to spread to every fresh orange.

Redundant Connection solution

Find the edge that creates a cycle in an otherwise tree-shaped graph.

Kth Largest Element in an Array solution

Return the kth largest value without requiring a fully sorted array.

Top K Frequent Elements solution

Return the k values that occur most often in an integer array.

Subsets solution

Generate every subset in the power set of a distinct integer array.

Invert Binary Tree solution

Swap every binary-tree node's left and right children.

Same Tree solution

Determine whether two binary trees have identical structure and values.

Validate Binary Search Tree solution

Check whether a binary tree satisfies strict BST ordering.

Binary Tree Level Order Traversal solution

Return binary-tree values grouped by breadth-first level.

Lowest Common Ancestor of a BST solution

Find the lowest shared ancestor of two values in a binary search tree.

Merge Two Sorted Lists solution

Merge two sorted linked lists by rewiring their nodes.

Linked List Cycle solution

Detect a cycle from a linked list represented by next-node indices.

Reorder List solution

Reorder a linked list by alternating nodes from its front and back.

Clone Graph solution

Deep-copy a connected graph from its adjacency-list representation.

Pacific Atlantic Water Flow solution

Find grid cells that can flow to both oceans under height constraints.

Network Delay Time solution

Compute how long a weighted directed network takes to receive a signal.

Merge K Sorted Lists solution

Merge k sorted sequences using a heap of current candidates.

Find Median from Data Stream solution

Return the median after incrementally inserting every stream value.

Permutations solution

Generate every ordering of a distinct integer array.

Combination Sum solution

Find target-sum combinations when candidate values may be reused.

Word Search solution

Find a word by backtracking through adjacent grid cells.

Implement Trie solution

Execute insert, exact-search, and prefix-search operations on a trie.

Design Add and Search Words solution

Build a trie that supports exact letters and wildcard searches.

LRU Cache solution

Simulate a bounded least-recently-used key-value cache.

Unique Paths solution

Count distinct top-left to bottom-right grid paths moving only right or down.

Pascal's Triangle solution

Build the first numRows rows of Pascal's triangle by summing adjacent values.

Count Primes solution

Count prime numbers strictly below n with the Sieve of Eratosthenes.

Sqrt(x) solution

Return the floored integer square root without a built-in sqrt function.

Happy Number solution

Decide whether summing squared digits eventually reaches one or cycles.

Excel Sheet Column Number solution

Convert a spreadsheet column title like AB into its 1-based number.

Insert Interval solution

Insert one interval into sorted disjoint intervals and merge any overlaps.

Non-overlapping Intervals solution

Remove the fewest intervals so the remaining schedule has no overlaps.

Sort Colors solution

Sort an array of 0s, 1s, and 2s in one in-place pass.

Number of Provinces solution

Count how many disconnected city groups exist in an undirected connectivity matrix.

Minimum Size Subarray Sum solution

Find the shortest contiguous subarray whose sum reaches a target.

3Sum solution

Find every unique triplet of numbers in an array that sums to zero.

Rotate Image solution

Rotate an n x n matrix 90 degrees clockwise and return the rotated matrix.

Search in Rotated Sorted Array solution

Find a target value's index in an ascending array that has been rotated at an unknown pivot.

Longest Consecutive Sequence solution

Find the length of the longest run of consecutive integers in an unsorted array.

Sum of Two Integers solution

Add two integers together without using the + or - operators.

Symmetric Tree solution

Check whether a binary tree is a mirror image of itself around its center.

Diameter of Binary Tree solution

Find the length in edges of the longest path between any two nodes in a binary tree.

Kth Smallest Element in a BST solution

Return the k-th smallest value stored in a binary search tree.

Add Two Numbers solution

Add two non-negative integers represented as reversed-digit linked lists.

Remove Nth Node From End of List solution

Delete the n-th node from the end of a singly linked list in one pass.

Path Sum solution

Determine whether a binary tree has a root-to-leaf path whose values sum to a target.

Convert Sorted Array to Binary Search Tree solution

Build a height-balanced binary search tree from an ascending array of unique integers.

Reverse Linked List II solution

Reverse only the nodes between two 1-indexed positions of a singly linked list, in one pass.

Swap Nodes in Pairs solution

Swap every adjacent pair of nodes in a singly linked list without changing the node values.

Binary Tree Right Side View solution

Return the values visible when looking at a binary tree from the right side, ordered top to bottom.

Merge Two Binary Trees solution

Overlay two binary trees, summing values where both trees have a node.

Path Sum II solution

Find every root-to-leaf path in a binary tree whose values sum to a target.

Construct Binary Tree from Preorder & Inorder solution

Rebuild a unique binary tree from its preorder and inorder traversal arrays.

Linked List Cycle II solution

Find the index where a cycle begins in a linked list represented by next-node indices, or -1 if there is none.

Odd Even Linked List solution

Regroup a singly linked list so all odd-positioned nodes come before all even-positioned nodes, preserving relative order within each group.

Task Scheduler solution

Find the minimum timeline length needed to schedule tasks with a cooldown between identical letters.

Connected Components in an Undirected Graph solution

Count how many disconnected components remain in an undirected graph with labeled vertices.

Palindrome Partitioning solution

Return every way to cut a string into palindromic substrings, ordered by DFS prefix exploration.

Word Search II solution

Find all dictionary words that can be traced through adjacent board cells without reusing a cell.

Word Ladder solution

Find the length of the shortest one-letter transformation sequence from a start word to an end word.

Daily Temperatures solution

Return how many days each temperature must wait until a warmer reading appears.

Decode Ways solution

Count how many valid A1Z26 decodings a digit string can represent.

Gas Station solution

Find the unique starting station that lets you complete a circular route, or report failure.

Alien Dictionary solution

Recover a valid alien letter order from a sorted word list, returning the lexicographically smallest valid order.

Binary Tree Maximum Path Sum solution

Find the largest sum obtainable from any non-empty path inside a binary tree.

Meeting Rooms II solution

Return the minimum number of meeting rooms needed so every interval can be scheduled without overlap.

Minimum Window Substring solution

Find the shortest substring of s that still contains every character required by t, including duplicates.

Longest Increasing Path in a Matrix solution

Find the maximum path length you can build by moving to orthogonally adjacent cells with strictly larger values.

Range Sum Query - Mutable solution

Process point updates and inclusive range-sum queries efficiently over the same integer array.

Cheapest Flights Within K Stops solution

Find the least expensive route from src to dst when you may use at most k intermediate stops.

Group Anagrams solution

Group strings that are anagrams of one another into deterministic buckets.

Min Stack solution

Process push, pop, top, and getMin operations on a stack in constant time.

Accounts Merge solution

Merge account rows that share an email address into deterministic combined profiles.

K Closest Points to Origin solution

Return the k points with the smallest distance from the origin in deterministic order.

Counting Bits solution

Return the number of set bits for every integer from 0 through n.