Coding interview problems

Two Sum

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

Merge Intervals

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

Reverse a String

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

Longest Common Prefix

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

Palindrome String

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

Palindromic Substrings

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

Longest Palindromic Substring

Return the longest contiguous substring that is a palindrome.

Palindrome Linked List

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

Roman to Integer

Convert a valid canonical Roman numeral into its integer value.

Letter Combinations of a Phone Number

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

Valid Parentheses

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

Find the Index of the First Occurrence in a String

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

Length of Last Word

Measure the final whitespace-delimited word in a string.

Spiral Matrix

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

FizzBuzz

Produce the classic FizzBuzz sequence using divisibility rules.

Contains Duplicate

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

Maximum Subarray

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

Fibonacci Number

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

Factorial

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

Move Zeroes

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

Single Number

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

Best Time to Buy and Sell Stock

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

Valid Anagram

Check whether two lowercase strings contain identical character frequencies.

Binary Search

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

Count Vowels

Count ASCII vowels in a string without changing other characters.

Climbing Stairs

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

Product of Array Except Self

Build prefix and suffix products without division.

Trapping Rain Water

Compute water trapped between elevation bars after rainfall.

Edit Distance

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

Median of Two Sorted Arrays

Find the combined median without fully merging two sorted arrays.

Longest Valid Parentheses

Measure the longest contiguous substring of balanced parentheses.

Palindrome Number

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

Plus One

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

Majority Element

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

Is Subsequence

Check whether one string is a subsequence of another.

First Unique Character in a String

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

Search Insert Position

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

House Robber

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

Jump Game

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

Coin Change

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

Longest Substring Without Repeating Characters

Find the length of the longest substring without repeating characters.

Container With Most Water

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

Number of Islands

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

Word Break

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

Longest Increasing Subsequence

Find the length of the longest strictly increasing subsequence.

Largest Rectangle in Histogram

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

Sliding Window Maximum

Return the maximum of every contiguous window of size k.

Jump Game II

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

Reverse Linked List

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

Maximum Depth of Binary Tree

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

Course Schedule

Determine whether all courses can be completed given prerequisite pairs.

Rotting Oranges

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

Redundant Connection

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

Kth Largest Element in an Array

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

Top K Frequent Elements

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

Subsets

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

Invert Binary Tree

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

Same Tree

Determine whether two binary trees have identical structure and values.

Validate Binary Search Tree

Check whether a binary tree satisfies strict BST ordering.

Binary Tree Level Order Traversal

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

Lowest Common Ancestor of a BST

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

Merge Two Sorted Lists

Merge two sorted linked lists by rewiring their nodes.

Linked List Cycle

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

Reorder List

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

Clone Graph

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

Pacific Atlantic Water Flow

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

Network Delay Time

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

Merge K Sorted Lists

Merge several sorted linked lists into one sorted list.

Find Median from Data Stream

Return the median after incrementally inserting every stream value.

Permutations

Generate every ordering of a distinct integer array.

Combination Sum

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

Word Search

Find a word by backtracking through adjacent grid cells.

Implement Trie

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

Design Add and Search Words

Add words to a dictionary and search with exact letters or single-letter wildcards.

LRU Cache

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

Unique Paths

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

Pascal's Triangle

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

Count Primes

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

Sqrt(x)

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

Happy Number

Decide whether summing squared digits eventually reaches one or cycles.

Excel Sheet Column Number

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

Insert Interval

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

Non-overlapping Intervals

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

Sort Colors

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

Number of Provinces

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

Minimum Size Subarray Sum

Find the shortest contiguous subarray whose sum reaches a target.

3Sum

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

Rotate Image

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

Search in Rotated Sorted Array

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

Longest Consecutive Sequence

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

Sum of Two Integers

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

Symmetric Tree

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

Diameter of Binary Tree

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

Kth Smallest Element in a BST

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

Add Two Numbers

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

Remove Nth Node From End of List

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

Path Sum

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

Convert Sorted Array to Binary Search Tree

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

Reverse Linked List II

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

Swap Nodes in Pairs

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

Binary Tree Right Side View

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

Merge Two Binary Trees

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

Path Sum II

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

Construct Binary Tree from Preorder & Inorder

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

Linked List Cycle II

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

Reorder a linked list so odd-positioned nodes come first, keeping each group's relative order.

Task Scheduler

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

Connected Components in an Undirected Graph

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

Palindrome Partitioning

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

Word Search II

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

Word Ladder

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

Daily Temperatures

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

Decode Ways

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

Gas Station

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

Alien Dictionary

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

Binary Tree Maximum Path Sum

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

Meeting Rooms II

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

Minimum Window Substring

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

Longest Increasing Path in a Matrix

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

Range Sum Query - Mutable

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

Cheapest Flights Within K Stops

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

Group Anagrams

Group anagrams while preserving the original order of words and their groups.

Min Stack

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

Accounts Merge

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

K Closest Points to Origin

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

Counting Bits

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

Maximum Product Subarray

Find the largest product obtainable from any non-empty contiguous subarray.

Course Schedule II

Return a valid topological ordering of courses, or an empty array if a cycle blocks completion.

Kth Smallest Element in a Sorted Matrix

Return the kth value in an n-by-n matrix whose rows and columns are both sorted ascending.

Missing Number

Find the only value missing from the distinct numbers drawn from the range 0 through n.

Find All Anagrams in a String

Return every starting index where p appears as an anagram inside s.

Find Minimum in Rotated Sorted Array

Find the smallest value in an ascending array that has been rotated an unknown number of times.

Search a 2D Matrix

Decide whether a target value exists in a matrix whose rows and row-boundaries are both sorted ascending.

Single Number II

Find the one value that appears a single time when every other value appears exactly three times.

Subarray Sum Equals K

Count how many contiguous subarrays add up to exactly the target value k.

Isomorphic Strings

Determine whether two strings share a consistent one-to-one character mapping in both directions.

Next Permutation

Return the lexicographically next greater permutation, or wrap to ascending order.

First Missing Positive

Find the smallest positive integer that does not appear in an unsorted array.

Set Matrix Zeroes

Zero out the entire row and column of every cell that contains a zero.

Spiral Matrix II

Generate an n-by-n matrix filled with 1 through n² in clockwise spiral order.

Balanced Binary Tree

Check whether every binary-tree node stays within one level of balance.

Lowest Common Ancestor of a Binary Tree

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

Binary Tree Zigzag Level Order Traversal

Return binary-tree values level by level, alternating left-to-right and right-to-left order.

Pseudo-Palindromic Paths in a Binary Tree

Count root-to-leaf paths whose digits can be rearranged to form a palindrome.

N-Queens

Return all valid N-Queens boards in deterministic DFS placement order.

Rotate String

Check whether one string can be rotated to become another.