Coding Interview Problems

Generate all possible T9 text strings from phone keypad digit presses using DFS backtracking. A classic recursion and combinations interview problem.

Given a string of unique characters, return all permutations of all combinations. Solve it with recursion and backtracking.

Find the maximum sum path from top-left to bottom-right in a grid using dynamic programming with O(m*n) time and space.

Find the maximum sum path from top-left to bottom-right in a grid using DFS, moving only down or right.

Given a binary tree and two node values, find the minimum number of edges between them using the Lowest Common Ancestor (LCA) approach.

Given a Boggle board and a target word, search the board using DFS with backtracking to determine if the word can be formed by adjacent cells.

Given an array of 9 unique integers from 1 to 10 with one missing, find the missing number using the sum formula in O(n) time and O(1) space.

Given k sorted linked lists, merge them into one globally sorted list using a min heap for O(n * k * log(k)) time and O(k) space.

Given the root of a binary tree and a target value, find the minimum distance from the root to the node containing the target. A DFS approach runs in O(n) time.

Learn how to serialize a binary tree to a string and deserialize it back using recursive pre-order traversal. Covers O(n) time and space solution.

Given a compressed, serialized binary tree string, decompress and reconstruct the original tree using level-order traversal with a queue.

Given a 2D board of letters, find all paths from top-left to bottom-right moving only down or right, returning each path as a string.

Learn how to implement a Trie data structure by building the insert method. Step-by-step walkthrough with visualizations.

Learn how to implement the search method in a Trie data structure, distinguishing complete words from prefixes using word boundaries.

Learn how to merge overlapping intervals using sorting and linear scan. Step-by-step walkthrough with visualizations and solutions in 14 languages.

Given the root of a binary tree, determine if it is a valid BST using an iterative approach with a queue and bounds tracking.

Find the maximum sum path between any two nodes in a binary tree that contains both positive and negative values. A classic Google and Facebook interview question.

Given a square matrix representing a black and white image, rotate it clockwise by 90 degrees using transpose and vertical flip.

Rotate a square 2D matrix 90 degrees counterclockwise using the transpose-then-flip technique. O(m*n) time, O(m*n) space.

Given an m x n integer matrix, return its elements visited in clockwise spiral order starting from the top left. O(m*n) time and space.

Find the maximum sum path between any two nodes in a binary tree using post-order recursion with a pivot-node technique.

Given a BST and two integers a and b, return all node values in the inclusive range [a, b] in sorted order using optimized in-order traversal.

Given a BST and integer k, find the kth largest node using in-order traversal. A classic BST problem asked at Twitter and Facebook.

Given the root of a BST and integer k, find and return the data of the kth smallest node using in-order traversal.

Given the head of a singly linked list, insert a new node at the front and return the new head. A fundamental O(1) linked list operation.

Given the root of a BST and an integer n, insert a new node with data n and return the root without modifying existing node positions.

Given a binary tree and a target node, find all ancestors from closest to farthest using DFS with backtracking.

Sum every node in a binary tree using an iterative BFS approach with a queue, no recursion allowed. Walkthrough, complexity, and solutions in 12 languages.

Given the root of a binary tree, count the number of half nodes - nodes with exactly one child. Solved with a clean recursive approach using XOR.

Given the root of a binary tree, count the number of full nodes where both left and right children exist. A classic recursion problem asked at Microsoft and LinkedIn.

Find the deepest rightmost node of a binary tree using BFS level-order traversal with a queue in O(n) time and O(n) space.

Given a binary tree, collect its node values level by level into a nested list using BFS with a queue. O(n) time, O(n) space.

Write a method to insert a node at the end of a singly linked list and return the head of the modified list.

Given a binary tree root, traverse nodes level by level from left to right using BFS and return their values as a list.

Learn how to iteratively traverse a binary tree in-order using a stack. Step-by-step walkthrough with visualizations, complexity analysis, and solutions in 14 languages.

Given the head of a doubly linked list, delete the nth node and return the modified list. Uses a sentinel node for clean edge case handling.

Given the head of a doubly linked list, an integer n, and an index i, insert a new node with data n at index i. Return the modified list head.

Given the root of a binary tree, find the maximum data value using an iterative BFS approach with a queue. O(n) time and space.

Given the head of a singly linked list, insert a new node with value n at zero-indexed position i. Return the modified list head.

Given closing prices over n days, find the maximum profit from unlimited buy-sell transactions. A greedy algorithm problem frequently asked at Facebook and Bloomberg.

Given the root of a binary search tree, find the maximum data value by traversing to the rightmost node in O(log n) time.

Given the head of a doubly linked list and an integer, insert a new node at the head and return the modified list. O(1) time and space.

Given a square image as a 2D array, return its transpose by swapping rows and columns. A classic matrix manipulation problem.

Compute the Nth Fibonacci number in O(n) time and O(1) space using an iterative sliding window of two variables.

Given the root of a binary tree and a target value, iteratively search the tree using BFS to determine if a node with that value exists.

Given the root of a binary tree, return the sum of all node values using recursion. A clean intro to recursive tree accumulation.

Given the root of a binary tree, count and return the number of leaf nodes. A leaf node has no children. Solved with recursion in O(n) time.

Given the root of a binary tree, return a list of node values visited in pre-order traversal using recursion.

Given the root of a binary tree, recursively find and return the maximum data value of any node. Covers DFS, base cases, and O(n) time complexity.

Given the root of a binary tree, return its size - the total number of nodes it contains. A classic recursion problem frequently asked at Oracle and Cisco.

Given the root of a binary tree, return its node values using iterative pre-order traversal with a stack in O(n) time.

Recursively raise a number to a power in O(log n) time without using Math.pow or the ^ operator. Handles negative exponents.

Learn how to implement bubble sort, a classic sorting algorithm that repeatedly swaps adjacent elements to sort an array in-place with O(n^2) time and O(1) space.

Write a recursive method to raise a number to a power without using Math.pow or the ^ operator. Understand the O(n) time and space recursive approach.

Check if a given integer is a palindrome using constant space by reversing it digit by digit with modulo and division.

Learn how to recursively delete the nth node from a linked list using a clean recursive approach with O(n) time and O(n) space complexity.

Given the head of a singly linked list, delete the node at the nth index and return the modified list. Handles edge cases like out-of-bounds indices.

Compress a sorted string by replacing consecutive repeated characters with the character and its count. A classic string manipulation interview question.

Write a method that checks if two strings are anagrams using a character frequency map. O(n) time, O(n) space.

Convert a decimal integer to its minimal binary string representation using recursion. A classic problem testing recursive thinking and base conversion.

Given a DNA string, insert a G between every pair of adjacent identical letters using recursion. Walkthrough, complexity analysis, and solutions in 14 languages.

Place n queens on an n x n chessboard so no two attack each other. Learn the backtracking approach with visual walkthroughs.

Generate all distinct permutations of an array that may contain duplicates using backtracking with duplicate pruning.

Generate all possible permutations of an array of distinct integers using backtracking. A staple at Meta and Bloomberg.

Given a string and a pattern with ? and * wildcards, determine if the pattern matches the entire string using dynamic programming.

Multiply two non-negative integers represented as strings without converting them to integers. Uses the grade-school multiplication algorithm.

Find all unique combinations from a list of candidates that sum to a target, using each number at most once. A classic backtracking problem.

Generate the nth term of the count-and-say sequence using iterative run-length encoding. A favorite at Apple, Meta, and Akuna Capital.

Solve a Sudoku puzzle using backtracking by filling empty cells with digits 1-9 while respecting row, column, and 3x3 sub-grid constraints.

Validate a partially filled 9x9 Sudoku board by checking rows, columns, and 3x3 sub-boxes for duplicate digits using HashSets.

Given a sorted array, find the first and last positions of a target value using binary search in O(log n) time.

Find the length of the longest valid, well-formed parentheses substring using a two-pass counter technique in O(n) time and O(1) space.

Rearrange an integer array into the lexicographically next greater permutation in-place with O(1) extra memory.

Find all starting indices of substrings that are concatenations of all given words using a sliding window and hash map approach.

Divide two integers without using multiplication, division, or modulus. Uses bit shifting for O(log² n) time.

Given two strings needle and haystack, find the index where needle first appears in haystack. Return -1 if not found. O(n*m) brute force solution.

Swap every two adjacent nodes in a linked list without modifying values, using a dummy node and pointer manipulation in O(n) time and O(1) space.

Given a string, find and return the first character that appears only once. Solve it in O(n) time using a frequency map with insertion order.

Given two sorted linked lists, merge them into a single sorted list by splicing nodes together. O(n) time, O(1) space using a dummy node.

Find all unique quadruplets in an array that sum to a target value using sorting and the two-pointer technique in O(n^3) time.

Given an integer array and a target, find three numbers whose sum is closest to the target. Solve it with sorting and two pointers in O(n^2) time.

Convert an integer to its Roman numeral representation using a greedy approach with parallel value and symbol arrays.

Implement regex matching with '.' and '*' support using dynamic programming. A classic hard DP problem asked at Google, Amazon, and Microsoft.

Determine if an integer is a palindrome without converting it to a string. Reverse only half the digits for an O(log n) solution with O(1) space.

Transform a string into a zigzag pattern across N rows, then read it row by row. A classic string manipulation problem asked at Google, Apple, and Amazon.

Flip a 2D integer array on its vertical axis to produce its mirror image. A beginner-friendly matrix problem with O(m*n) time.

Add two numbers represented as reversed linked lists, returning the sum as a new linked list. A classic interview problem at Apple, Google, and Meta.

Given a 2D array representing a grayscale image, write a method that returns a copy flipped on its horizontal axis. A classic matrix manipulation problem.

Given a singly linked list, write a method to delete its tail node and return the head of the modified list.

Determine if an array can be split into two halves with equal sum using recursion and subset sum reduction.

Given an array of positive integers and an integer k, find the maximum sum of any contiguous sub-array of size k using the sliding window technique.

Given a singly linked list, write a method to delete its head node and return the modified list. A simple but essential linked list operation.

Given a list of origin-destination flight pairs, build an adjacency list mapping each origin to its distinct reachable destinations.

Given a flight route graph as an adjacency list, determine if you can fly from an origin to a destination via non-stop or connecting flights using BFS.

Given a string of digits, return all valid IPv4 addresses that can be formed by inserting dots. Uses DFS backtracking with byte validation.

Given the roots of two binary trees, write a method to check if they are mirror images of each other using recursion.

Given the root of a binary tree, find the level with the maximum sum using BFS with two queues. O(n) time and space.

Write a method to find and replace all commas in a CSV string with a provided replacement string. A classic string manipulation warmup for coding interviews.

Determine whether an ASCII string has all unique characters using a HashSet for O(n) time and O(1) space.

Given a string of brackets, write a method that checks if every opening bracket has a matching closing bracket in the correct order. A classic stack-based interview problem.

Given a list of positive integers and a target, determine if any subset sums to the target. A classic recursion and backtracking interview problem.

Given an input string of unique characters, return all permutations of the string using recursion and backtracking. A classic recursion problem testing divide-and-conquer thinking and the insert-at-every-position technique.

Given the root of a binary tree and two target node values, find and return the data of the lowest common ancestor. A classic binary tree recursion problem frequently asked at Facebook, Amazon, and Microsoft.

Given an integer array, write a method that identifies and returns all numbers that appear more than once as a sorted array containing each duplicate value exactly once.

Write a method that checks if a given positive integer is a happy number by repeatedly summing the squares of its digits until it reaches 1 or enters a cycle. A classic set-based cycle detection interview problem.

Given the root of a binary tree, find its diameter - the number of nodes on the longest path between any two nodes. A classic tree recursion problem testing depth-first search and height computation.

Learn how to convert a binary tree to its mirror image by recursively swapping left and right children at every node. Walkthrough with visualizations and solutions in 14 languages.

Given the roots of two binary trees, determine if they are identical. Two trees are identical when they share the same shape and the same data at every node. A classic recursion problem testing base cases and tree traversal.

Given the head of a singly linked list, remove all duplicate nodes while keeping the first occurrence of each value. A classic linked list problem testing set-based traversal and pointer manipulation.

Rotate an array to the left by k positions in-place using the three-reversal trick. A classic array manipulation interview problem with O(n) time and O(1) space.

Check whether a singly linked list is a palindrome using constant space. Learn the slow/fast pointer technique to find the middle, reverse the second half in-place, and compare both halves node by node.

Given the root of a binary tree, find the minimum depth - the smallest number of node traversals from root to the nearest leaf. A classic BFS / level order traversal problem using a two-queue approach.

Write a recursive method to return the nth Fibonacci number. A classic recursion problem testing base cases and the Fibonacci recurrence relation.

Write a method to test whether a given positive integer is a power of 2 using bitwise operations, with constant time and space complexity.

Search a sorted array for a target value using the binary search algorithm. O(log n) time, O(1) space. A foundational divide-and-conquer interview problem.

Write a method that reverses an input integer using only constant space. A fundamental number manipulation problem that tests your understanding of modular arithmetic and integer division.

Write a method that merges two sorted arrays into a single globally sorted array using a two-pointer approach. A fundamental sorting and array interview problem.

Write a method that checks if a string reads the same forward and backward, ignoring case. A classic string manipulation interview problem.

Given an array of stock prices over n days, find the maximum profit from a single buy-sell transaction. A classic array problem that tests greedy algorithm thinking.

Write a method to find and return the data of the middle node of a singly linked list in one pass using the slow and fast pointer technique.

Given an array that contains duplicates and exactly one unique number, write a method that finds and returns the unique number using a HashMap for O(n) time complexity.

Given an array of integers and an integer k, find the average of all contiguous subarrays of size k using the sliding window technique in O(n) time.

Given an integer triangle as a nested list, find the minimum sum path from the top to the bottom. A classic dynamic programming interview problem solved with bottom-up DP in O(n) space.

Given a singly-linked list, reverse the list in pairs by swapping adjacent nodes two at a time and return the head of the modified list.

Given a list of sorted, non-overlapping intervals and a new interval, write a method to insert it into the list such that there is no overlap between them (merge as needed) and the sorted order is maintained.

Given n pairs of parentheses, generate all valid combinations using recursive backtracking. A classic recursion problem frequently asked at Amazon, Google, and Facebook.

Find the length of the longest strictly increasing subsequence using binary search and patience sorting. O(n log n) time, O(n) space, with step-by-step walkthrough.

Given an array of integers and a target, find indices of two numbers that add up to the target using a hash map in O(n) time.

Learn to check if two strings are anagrams using character frequency counting. Walkthrough with visualizations and solutions in 14 languages.

Build an output array where each element is the product of every other element, without using division. O(n) time using prefix and suffix products.

Find the lowest common ancestor of two nodes in a BST using its ordering property. O(h) time, O(1) space.

Given the root of a binary search tree and an integer k, find and return the kth smallest value among all the node values in the tree. The indexing is 1-based.

Given an array of integers with length n, identify all elements that occur more than n/3 times. Solve it in O(n) time and O(1) space using the extended Boyer-Moore Voting Algorithm.

Determine if all courses can be completed given prerequisites. DFS cycle detection in O(V+E) time using three-state coloring.

Maximize the sum of non-adjacent elements using space-optimized DP. O(n) time, O(1) space walkthrough of the House Robber problem.

Given a positive integer n, create a function to determine how many set bits (1s) are present in its binary form, also referred to as the Hamming weight.

Find two numbers in a sorted array that sum to a target using the two-pointer technique. O(n) time, O(1) space.

Find the minimum element in a rotated sorted array using binary search. O(log n) time, O(1) space, with step-by-step walkthrough and solutions in 14 languages.

Find the contiguous subarray with the highest product using a max/min tracking algorithm. O(n) time, O(1) space, with step-by-step walkthrough and negative number handling.

Find all possible ways to segment a string into valid dictionary words using backtracking with memoization.

Learn to find the single number in an array where every other element appears twice, using XOR bit manipulation. Walkthrough with visualizations and solutions in 14 languages.

Determine if a string is a palindrome after removing non-alphanumeric characters and ignoring case. Two-pointer O(n) time, O(1) space solution.

Given the root of a binary tree, determine and return the highest possible path sum for any path that contains at least one node. A path consists of a sequence of nodes where each pair of consecutive nodes is connected by an edge, and no node appears more than once.

Given a binary tree and a target sum, determine if any root-to-leaf path sums to the target. Recursive DFS solution in O(n) time and O(h) space.

Given the root of a binary tree, determine its maximum depth. The maximum depth of a binary tree is defined as the number of nodes along the longest path from the root node to the most distant leaf node.

Determine if two binary trees are identical by comparing structure and node values using recursive DFS. O(n) time, O(n) space.

Given an m x n character grid and a target word, determine if the word exists by connecting adjacent cells using DFS with backtracking in O(m * n * 4^k) time.

Generate all possible subsets (the power set) of a unique integer array using backtracking and bit manipulation. O(2^n) time and space, with full recursion tree walkthrough and solutions in 14 languages.

Sort an array of 0s, 1s, and 2s in-place using the Dutch National Flag algorithm. O(n) time, O(1) space. Full walkthrough with solutions in 14 languages.

Given an m x n matrix, if any element is 0, set its entire row and column to 0 in place. Solve it in O(1) space by using the first row and column as markers.

Master the edit distance problem using 2D dynamic programming. Learn to compute the minimum insert, delete, and replace operations to transform one string into another.

Find the number of distinct ways to climb n stairs taking 1 or 2 steps at a time using Fibonacci-like dynamic programming.

Learn to merge overlapping intervals by sorting and scanning. Step-by-step walkthrough with visualizations and solutions in 14 languages.

Determine if you can reach the last array index using a greedy maxReach approach. O(n) time, O(1) space walkthrough.

Given an m x n matrix, return all elements in spiral order using four boundary pointers. O(m*n) time, O(m*n) space for the output.

Find the contiguous subarray with the largest sum using Kadane's algorithm. O(n) time, O(1) space, with step-by-step walkthrough.

Group an array of strings so that anagrams appear together using sorted keys and a hash map in O(n * k log k) time.

Rotate an n x n matrix 90 degrees clockwise in-place using the transpose-then-reverse technique. O(n^2) time, O(1) space.

Given an array of unique integers and a target, find all unique combinations that sum to the target. Each number can be reused unlimited times. Solved with backtracking in O(2^target) time.

Given a sorted array of unique integers and a target value, find the index of the target or the position where it should be inserted. O(log n) binary search solution with step-by-step walkthrough and solutions in 14 languages.

Search for a target in a rotated sorted array using modified binary search. O(log n) time, O(1) space, with step-by-step walkthrough.

Learn two approaches to merge k sorted linked lists: min-heap for O(N log k) efficiency and divide-and-conquer for elegant recursion. Complete solutions in 14 languages.

Learn to validate matching brackets using a stack. Step-by-step walkthrough of the Valid Parentheses problem with visualizations and solutions in 14 languages.

Given the head of a linked list, remove the nth node from the end and return its head. Solved with the two-pointer technique in O(n) time and O(1) space.

Given a string of digits from 2-9, generate all possible letter combinations using backtracking. A classic interview problem at Google, Amazon, Apple, and Meta.

Find all unique triplets in an array that sum to zero using sort and two pointers in O(n^2) time.

Find the longest common prefix shared by an array of strings using the horizontal scanning approach. O(n * m) time, O(1) space.

Convert a Roman numeral string to its integer value using a hash map and right-to-left traversal with subtraction rule detection. O(n) time, O(1) space.

Find two lines that hold the most water using the two-pointer technique in O(n) time and O(1) space.

Reverse the digits of a signed 32-bit integer, returning 0 if the result overflows. O(log n) time, O(1) space.

Given a string s, find and return the longest palindromic substring within s using the expand-around-center technique in O(n^2) time and O(1) space.

Find the median of two sorted arrays in O(log(min(m,n))) time using binary search partitioning. A classic hard problem tested at Google, Amazon, and Uber.

Given an m x n grid of 1s (land) and 0s (water), count the number of islands using DFS flood fill in O(m*n) time and space.

Given a string, find the length of the longest substring without repeated characters using the sliding window technique. O(n) time, O(n) space.

Traverse a binary tree bottom-up, level by level, returning node values from the deepest level to the root. Uses BFS with a queue and a final reversal for O(n) time and O(n) space.

Calculate the number of unique paths from top-left to bottom-right in an m×n grid, moving only right or down.

Master the art of validating parentheses with wildcards - a favorite interview problem at Amazon, Microsoft, and Apple that tests your greedy algorithm skills.

Find all words that appear immediately after a specific two-word sequence in a string - a classic string parsing challenge.

Given the head of a singly linked list, write a method that reverses the list and returns its new head.

Given the root of a binary search tree, find and return the minimum data value by traversing to the leftmost node using recursion.

Given the root TreeNode of a binary tree, write a method return its height. An empty tree has a height of 0 while a single node tree has a height of 1.