Technical Interview Questions and Solutions

Binary Array Partition

Given an array Z of 0s and 1s, divide the array into 3 non-empty parts, such that all of these parts represent the same binary value.

Print Folder Structure

Given a list of file paths, print all of the files in each of the folders.

Image Filter Service

Create a service that allows users to upload an image and apply filters and then sends users a link to download their filtered image.

Permutation in String

Given two strings s1 and s2, return true if s2 contains a permutation of s1, or false otherwise.

Maximum Subarray

Given an integer array nums, find the subarray with the largest sum, and return its sum.

LRU Cache

Implement an LRU Cache LRU = Least recently used cache

Find Peak Element in a 2D Array

Given a two-dimensional binary matrix where 1 represents water and 0 represents land, mutate the matrix in place and return the matrix with the highest peak maximized.

Simplify Path

You are given a path to a file as a string. The path can contain the symbols: “..” for the parent directory and “.” for the current directory. Convert the path into its simplified form.

Build a Max Heap From an Array

Given an array of integers, transform the array in-place to a max heap.

Even Odd Tree

Given a tree, verify that on even levels, all values in the level are strictly increasing and even. On odd levels, verify all values in the level are strictly decreasing and odd.

Subarray Sum Equals K

Given an unsorted array of integers and an integer k, find the number of subarrays whose sum equals k.

Longest Substring Without Repeating Characters

Given a string s, find the length of the longest substring without repeating characters.

Distributed Databases

How would you organize a SQL database like MySQL such that you can add more machines once your current ones reach maximum capacity? With the limitation that you do not have access to any automated tools for distributing.

Odd Even Linked List

Given the head of a singly linked list, group all the nodes with odd indices together followed by the nodes with even indices, and return the reordered list.

Generate Parentheses

Given `n` pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

Insert Delete getRandom O(1)

Design and implement an efficient sampler that works in average O(1) time complexity.

Recover Binary Search Tree

Two elements of a binary search tree (BST) are swapped by mistake. Recover the tree without changing its structure.

Three Sum

Given an array of integers, return an array of triplets such that i != j != k and nums[i] + nums[j] + nums[k] = 0.

Remove Nth Node from End of List

Given the head of a linked list, remove the nth node from the end of the list and return its head.

Employee Hierarchy

Given an array of employee IDs including who they report to, write a function to calculate the score for a given employee.

Kth Smallest Element

Given an integer array and an integer k, return the kth smallest element in the array.

Walls and Gates

You are given a m x n 2D grid initialized with these three possible values. Fill each empty room with the distance to its nearest gate.

K Closest Points To Origin

Given a list of tuples that represent (X, Y) coordinates on an XY plane and an integer K, return a list of the K-closest points to the origin (0, 0).

Reverse Nodes in k-Group

Given the head of a linked list, reverse the nodes of the list k at a time, and return the modified list.

Palindrome Generator

Print out all 8-digit palindromes. Limitation: We can't use string manipulation.

Valid Palindrome

Determine if this string, after removing any one character, can become a palindrome. If possible return true, otherwise return false.

Right View Of Binary Tree

Given the root of a binary tree, imagine yourself standing on the right side of it, return the values of the nodes you can see ordered from top to bottom.

XML Parser

Write an XML parser and formatter.

Fruit into Baskets

Given a sequence of fruit trees represented as an array of strings. Return the maximum number of fruit trees you can pick from given you can only have one type of fruit in each basket and once you start picking you can't skip a tree and then keep picking.

Reverse a Linked List

Given the head of a linked list, reverse the list and return the new head.

Currency Conversion

Given a set of parameters, find the conversion rate that maps to the 'from' currency to the 'to' currency from every single query. Your return value should be a number.

Count Complete Tree Nodes

Given the root of a complete binary tree, return the number of nodes in the tree.

Decode String

Given an encoded string, return its decoded string.

Sum Root to Leaf Numbers

You are given the root of a binary tree containing digits from 0 to 9 only. Each root-to-leaf path in the tree represents a number, for example, the root-to-leaf path 1 -> 2 -> 3 represents the number 123. Return the total sum of all root-to-leaf numbers.

Partition List

Given a list of integers L and a number K, write a function that reorganizes L into three partitions: elements less than K, elements equal to K, and elements greater than K. No additional lists may be used.

Reverse Words in a String

Given an input string `s`, reverse the order of the words without reversing the words themselves.

Copy List With Random Pointers

Given a linked list with nodes that have an additional pointer referring to another node in the list, return a deep copy of the list.

Longest Common Subsequence

Given two strings, return the longest common subsequence between the two strings.

Container With the Most Water

Given n non-negative integers, find two lines that form a container that can hold the most amount of water.

File Parsing

Given an inefficient file structure, how would you store data to efficiently look up the query? How would you alter this if you had many computers available?

Shuffle String

Write a function that takes a string as an input and returns a shuffled version of that string then write another function to analyze how well it was shuffled.

K Largest Elements

Write an efficient program for printing k largest elements in an array. Largest elements are returned in order largest to smallest.

Number of Subarrays with Bounded Maximum

Given an integer array nums and two integers left and right, return the number of contiguous non-empty subarrays such that the value of the maximum array element in that subarray is in the range [left, right].

Partition Equal Subset Sum

Given an array of positive numbers, determine if the array can be split such that the two partition sums are equal.

Partition to K Equal Sum Subsets

Given an integer array nums and an integer k, return true if it is possible to divide this array into k non-empty subsets whose sums are all equal.

Longest Palindromic Substring

Given a string s, return the longest palindromic substring in s.

Integer Replacement

Given an integer as an input, replace all the digits ‘0’ with ‘5’ in the integer.

Design WhatsApp

Design a message app system and client (e.g. WhatsApp) supporting a list of requirements.

Binary Tree Upside Down

Given a binary tree where every node has either 0 or 2 children and every right node is a leaf node, flip it upside down turning it into a binary tree where all left nodes are leaf nodes.

Lucky Numbers in a Matrix

Given an m x n matrix of distinct numbers, return all lucky numbers in the matrix in any order.

Confusing Number

Write a function that, given a room with 800 BIDDERS, identifies all the confusable numbers.

Longest Substring with At Most K Distinct Characters

Given a string, find the length of the longest substring in it with no more than K distinct characters.

Intersection of Linked List

Given the heads of two singly linked-lists headA and headB, return the node at which the two lists intersect.

Regular Expression Matching

Given an input string (s) and a pattern (p), implement regular expression matching with support for '.' and '*'. '.' Matches any single character. '*' Matches zero or more of the preceding element.

Design a Free Food App

Design the service for an app that supports distributing 6 million burgers in 10 minutes.

Minimum Cost to Construct String

Given a 2-D integer array mapping the letters ABCD and their costs. Calculate the smallest cost to make a string of length n.

Infinite Binary Print

Print out all numbers in binary, preserving leading zeros.

Prefix Pairs

Given a list of words, match all words with other words from the list that are a prefix for the word.

Number of Islands

Given a 2D matrix, where "1" represents land and "0" represents water, count how many islands are present.

Find Leaves of a Binary Tree

Given a binary tree, extract all the leaves in repeated succession into a list of lists by starting at the bottom and working your way upwards.

Top K Frequent Elements

Given a non-empty array of integers, return the k most frequent elements.

Reverse Integer

Given a 32-bit signed integer, reverse digits of the integer.

Most Frequent Element in an Array

Given an array of integers, find the most frequent element in the array. Write a method that takes an array of integers and returns an integer. If there is a tie, you can just return any.

Two Sum

Given an array of integers, return the indices of the two numbers that add up to a given target.

Boundary of Binary Tree

The boundary of a binary tree is the concatenation of the root, the left boundary, the leaves ordered from left-to-right, and the reverse order of the right boundary.

Alien Dictionary

You are given a list of lexicographically sorted words from an alien language. This language has a unique order. Return the alphabetical order of all the letters found in the list of words.

Transformation Dictionary

Given a dictionary of words, determine whether it is possible to transform a given word into another with a fixed number of characters.

Split Array Largest Sum

Given an integer array nums and an integer k, split nums into k non-empty subarrays such that the largest sum of any subarray is minimized. Return the minimized largest sum of the split.

Longest Increasing Path in a Matrix

Given an m x n integers matrix, return the length of the longest increasing path in the matrix. You may only move up, down, left, or right.

Find the Missing Number in an Array

Given an unsorted array of unique integers (size n + 1) and a first array identical to the second array, missing one integer (size n), find and output the missing integer.

Design LeetCode

Design a coding competition platform with a leaderboard and execution environment.

Meeting Rooms

Given a list of meetings, represented as tuples with a start and an end time, determine the minimum number of rooms required to schedule all the meetings.

Minimum Window Substring

Given two strings s and t of lengths m and n respectively, return the minimum window substring of s such that every character in t (including duplicates) is included in the window.

Reverse String

Write a program to reverse the given string.