Introduction

In this blog post, we'll share the most commonly asked coding interview questions at DoorDash. If you don't have months to study for your interviews, you can use AI tools like Chatmagic to generate solutions quickly and efficiently - helping you pass the interviews and get the job offer!

Problem #2: Longest Increasing Path in a Matrix

Given an m x n integers matrix, return the length of the longest increasing path in matrix. From each cell, you can either move in four directions: left, right, up, or down. You may not move diagonally or move outside the boundary (i.e., wrap-around is not allowed). Example 1: Input: matrix = [[9,9,4],[6,6,8],[2,1,1]] Output: 4 Explanation: The longest increasing path is [1, 2, 6, 9]. Example 2: Input: matrix = [[3,4,5],[3,2,6],[2,2,1]] Output: 4 Explanation: The longest increasing path is [3, 4, 5, 6]. Moving diagonally is not allowed. Example 3: Input: matrix = [[1]] Output: 1 Constraints: m == matrix.length n == matrix[i].length 1 <= m, n <= 200 0 <= matrix[i][j] <= 231 - 1

Topics: Array, Dynamic Programming, Depth-First Search, Breadth-First Search, Graph, Topological Sort, Memoization, Matrix

Problem #3: Binary Tree Maximum Path Sum

A path in a binary tree is a sequence of nodes where each pair of adjacent nodes in the sequence has an edge connecting them. A node can only appear in the sequence at most once. Note that the path does not need to pass through the root. The path sum of a path is the sum of the node's values in the path. Given the root of a binary tree, return the maximum path sum of any non-empty path. Example 1: Input: root = [1,2,3] Output: 6 Explanation: The optimal path is 2 -> 1 -> 3 with a path sum of 2 + 1 + 3 = 6. Example 2: Input: root = [-10,9,20,null,null,15,7] Output: 42 Explanation: The optimal path is 15 -> 20 -> 7 with a path sum of 15 + 20 + 7 = 42. Constraints: The number of nodes in the tree is in the range [1, 3 * 104]. -1000 <= Node.val <= 1000

Topics: Dynamic Programming, Tree, Depth-First Search, Binary Tree

Problem #4: Maximum Profit in Job Scheduling

We have n jobs, where every job is scheduled to be done from startTime[i] to endTime[i], obtaining a profit of profit[i]. You're given the startTime, endTime and profit arrays, return the maximum profit you can take such that there are no two jobs in the subset with overlapping time range. If you choose a job that ends at time X you will be able to start another job that starts at time X. Example 1: Input: startTime = [1,2,3,3], endTime = [3,4,5,6], profit = [50,10,40,70] Output: 120 Explanation: The subset chosen is the first and fourth job. Time range [1-3]+[3-6] , we get profit of 120 = 50 + 70. Example 2: Input: startTime = [1,2,3,4,6], endTime = [3,5,10,6,9], profit = [20,20,100,70,60] Output: 150 Explanation: The subset chosen is the first, fourth and fifth job. Profit obtained 150 = 20 + 70 + 60. Example 3: Input: startTime = [1,1,1], endTime = [2,3,4], profit = [5,6,4] Output: 6 Constraints: 1 <= startTime.length == endTime.length == profit.length <= 5 * 104 1 <= startTime[i] < endTime[i] <= 109 1 <= profit[i] <= 104

Topics: Array, Binary Search, Dynamic Programming, Sorting

Problem #5: Search Suggestions System

You are given an array of strings products and a string searchWord. Design a system that suggests at most three product names from products after each character of searchWord is typed. Suggested products should have common prefix with searchWord. If there are more than three products with a common prefix return the three lexicographically minimums products. Return a list of lists of the suggested products after each character of searchWord is typed. Example 1: Input: products = ["mobile","mouse","moneypot","monitor","mousepad"], searchWord = "mouse" Output: [["mobile","moneypot","monitor"],["mobile","moneypot","monitor"],["mouse","mousepad"],["mouse","mousepad"],["mouse","mousepad"]] Explanation: products sorted lexicographically = ["mobile","moneypot","monitor","mouse","mousepad"]. After typing m and mo all products match and we show user ["mobile","moneypot","monitor"]. After typing mou, mous and mouse the system suggests ["mouse","mousepad"]. Example 2: Input: products = ["havana"], searchWord = "havana" Output: [["havana"],["havana"],["havana"],["havana"],["havana"],["havana"]] Explanation: The only word "havana" will be always suggested while typing the search word. Constraints: 1 <= products.length <= 1000 1 <= products[i].length <= 3000 1 <= sum(products[i].length) <= 2 * 104 All the strings of products are unique. products[i] consists of lowercase English letters. 1 <= searchWord.length <= 1000 searchWord consists of lowercase English letters.

Topics: Array, String, Binary Search, Trie, Sorting, Heap (Priority Queue)

Problem #6: Koko Eating Bananas

Koko loves to eat bananas. There are n piles of bananas, the ith pile has piles[i] bananas. The guards have gone and will come back in h hours. Koko can decide her bananas-per-hour eating speed of k. Each hour, she chooses some pile of bananas and eats k bananas from that pile. If the pile has less than k bananas, she eats all of them instead and will not eat any more bananas during this hour. Koko likes to eat slowly but still wants to finish eating all the bananas before the guards return. Return the minimum integer k such that she can eat all the bananas within h hours. Example 1: Input: piles = [3,6,7,11], h = 8 Output: 4 Example 2: Input: piles = [30,11,23,4,20], h = 5 Output: 30 Example 3: Input: piles = [30,11,23,4,20], h = 6 Output: 23 Constraints: 1 <= piles.length <= 104 piles.length <= h <= 109 1 <= piles[i] <= 109

Topics: Array, Binary Search

Problem #7: Find K Closest Elements

Given a sorted integer array arr, two integers k and x, return the k closest integers to x in the array. The result should also be sorted in ascending order. An integer a is closer to x than an integer b if: |a - x| < |b - x|, or |a - x| == |b - x| and a < b Example 1: Input: arr = [1,2,3,4,5], k = 4, x = 3 Output: [1,2,3,4] Example 2: Input: arr = [1,1,2,3,4,5], k = 4, x = -1 Output: [1,1,2,3] Constraints: 1 <= k <= arr.length 1 <= arr.length <= 104 arr is sorted in ascending order. -104 <= arr[i], x <= 104

Topics: Array, Two Pointers, Binary Search, Sliding Window, Sorting, Heap (Priority Queue)

Problem #8: Count Nodes With the Highest Score

There is a binary tree rooted at 0 consisting of n nodes. The nodes are labeled from 0 to n - 1. You are given a 0-indexed integer array parents representing the tree, where parents[i] is the parent of node i. Since node 0 is the root, parents[0] == -1. Each node has a score. To find the score of a node, consider if the node and the edges connected to it were removed. The tree would become one or more non-empty subtrees. The size of a subtree is the number of the nodes in it. The score of the node is the product of the sizes of all those subtrees. Return the number of nodes that have the highest score. Example 1: Input: parents = [-1,2,0,2,0] Output: 3 Explanation: - The score of node 0 is: 3 1 = 3 - The score of node 1 is: 4 = 4 - The score of node 2 is: 1 1 2 = 2 - The score of node 3 is: 4 = 4 - The score of node 4 is: 4 = 4 The highest score is 4, and three nodes (node 1, node 3, and node 4) have the highest score. Example 2: Input: parents = [-1,2,0] Output: 2 Explanation: - The score of node 0 is: 2 = 2 - The score of node 1 is: 2 = 2 - The score of node 2 is: 1 1 = 1 The highest score is 2, and two nodes (node 0 and node 1) have the highest score. Constraints: n == parents.length 2 <= n <= 105 parents[0] == -1 0 <= parents[i] <= n - 1 for i != 0 parents represents a valid binary tree.

Topics: Array, Tree, Depth-First Search, Binary Tree

Problem #9: Minimum Number of Steps to Make Two Strings Anagram

You are given two strings of the same length s and t. In one step you can choose any character of t and replace it with another character. Return the minimum number of steps to make t an anagram of s. An Anagram of a string is a string that contains the same characters with a different (or the same) ordering. Example 1: Input: s = "bab", t = "aba" Output: 1 Explanation: Replace the first 'a' in t with b, t = "bba" which is anagram of s. Example 2: Input: s = "leetcode", t = "practice" Output: 5 Explanation: Replace 'p', 'r', 'a', 'i' and 'c' from t with proper characters to make t anagram of s. Example 3: Input: s = "anagram", t = "mangaar" Output: 0 Explanation: "anagram" and "mangaar" are anagrams. Constraints: 1 <= s.length <= 5 * 104 s.length == t.length s and t consist of lowercase English letters only.

Topics: Hash Table, String, Counting

Problem #10: Buddy Strings

Given two strings s and goal, return true if you can swap two letters in s so the result is equal to goal, otherwise, return false. Swapping letters is defined as taking two indices i and j (0-indexed) such that i != j and swapping the characters at s[i] and s[j]. For example, swapping at indices 0 and 2 in "abcd" results in "cbad". Example 1: Input: s = "ab", goal = "ba" Output: true Explanation: You can swap s[0] = 'a' and s[1] = 'b' to get "ba", which is equal to goal. Example 2: Input: s = "ab", goal = "ab" Output: false Explanation: The only letters you can swap are s[0] = 'a' and s[1] = 'b', which results in "ba" != goal. Example 3: Input: s = "aa", goal = "aa" Output: true Explanation: You can swap s[0] = 'a' and s[1] = 'a' to get "aa", which is equal to goal. Constraints: 1 <= s.length, goal.length <= 2 * 104 s and goal consist of lowercase letters.

Topics: Hash Table, String