Introduction

In this blog post, we'll share the most commonly asked coding interview questions at Affirm. 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 #1: Insert Delete GetRandom O(1)

Implement the RandomizedSet class: RandomizedSet() Initializes the RandomizedSet object. bool insert(int val) Inserts an item val into the set if not present. Returns true if the item was not present, false otherwise. bool remove(int val) Removes an item val from the set if present. Returns true if the item was present, false otherwise. int getRandom() Returns a random element from the current set of elements (it's guaranteed that at least one element exists when this method is called). Each element must have the same probability of being returned. You must implement the functions of the class such that each function works in average O(1) time complexity. Example 1: Input ["RandomizedSet", "insert", "remove", "insert", "getRandom", "remove", "insert", "getRandom"] [[], [1], [2], [2], [], [1], [2], []] Output [null, true, false, true, 2, true, false, 2] Explanation RandomizedSet randomizedSet = new RandomizedSet(); randomizedSet.insert(1); // Inserts 1 to the set. Returns true as 1 was inserted successfully. randomizedSet.remove(2); // Returns false as 2 does not exist in the set. randomizedSet.insert(2); // Inserts 2 to the set, returns true. Set now contains [1,2]. randomizedSet.getRandom(); // getRandom() should return either 1 or 2 randomly. randomizedSet.remove(1); // Removes 1 from the set, returns true. Set now contains [2]. randomizedSet.insert(2); // 2 was already in the set, so return false. randomizedSet.getRandom(); // Since 2 is the only number in the set, getRandom() will always return 2. Constraints: -231 <= val <= 231 - 1 At most 2 * 105 calls will be made to insert, remove, and getRandom. There will be at least one element in the data structure when getRandom is called.

Topics: Array, Hash Table, Math, Design, Randomized

Problem #2: Shortest Uncommon Substring in an Array

You are given an array arr of size n consisting of non-empty strings. Find a string array answer of size n such that: answer[i] is the shortest substring of arr[i] that does not occur as a substring in any other string in arr. If multiple such substrings exist, answer[i] should be the lexicographically smallest. And if no such substring exists, answer[i] should be an empty string. Return the array answer. Example 1: Input: arr = ["cab","ad","bad","c"] Output: ["ab","","ba",""] Explanation: We have the following: - For the string "cab", the shortest substring that does not occur in any other string is either "ca" or "ab", we choose the lexicographically smaller substring, which is "ab". - For the string "ad", there is no substring that does not occur in any other string. - For the string "bad", the shortest substring that does not occur in any other string is "ba". - For the string "c", there is no substring that does not occur in any other string. Example 2: Input: arr = ["abc","bcd","abcd"] Output: ["","","abcd"] Explanation: We have the following: - For the string "abc", there is no substring that does not occur in any other string. - For the string "bcd", there is no substring that does not occur in any other string. - For the string "abcd", the shortest substring that does not occur in any other string is "abcd". Constraints: n == arr.length 2 <= n <= 100 1 <= arr[i].length <= 20 arr[i] consists only of lowercase English letters.

Topics: Array, Hash Table, String, Trie

Problem #3: Group Anagrams

Given an array of strings strs, group the anagrams together. You can return the answer in any order. Example 1: Input: strs = ["eat","tea","tan","ate","nat","bat"] Output: [["bat"],["nat","tan"],["ate","eat","tea"]] Explanation: There is no string in strs that can be rearranged to form "bat". The strings "nat" and "tan" are anagrams as they can be rearranged to form each other. The strings "ate", "eat", and "tea" are anagrams as they can be rearranged to form each other. Example 2: Input: strs = [""] Output: [[""]] Example 3: Input: strs = ["a"] Output: [["a"]] Constraints: 1 <= strs.length <= 104 0 <= strs[i].length <= 100 strs[i] consists of lowercase English letters.

Topics: Array, Hash Table, String, Sorting

Problem #4: Valid Anagram

Given two strings s and t, return true if t is an anagram of s, and false otherwise. Example 1: Input: s = "anagram", t = "nagaram" Output: true Example 2: Input: s = "rat", t = "car" Output: false Constraints: 1 <= s.length, t.length <= 5 * 104 s and t consist of lowercase English letters. Follow up: What if the inputs contain Unicode characters? How would you adapt your solution to such a case?

Topics: Hash Table, String, Sorting

Problem #5: Insert Delete GetRandom O(1) - Duplicates allowed

RandomizedCollection is a data structure that contains a collection of numbers, possibly duplicates (i.e., a multiset). It should support inserting and removing specific elements and also reporting a random element. Implement the RandomizedCollection class: RandomizedCollection() Initializes the empty RandomizedCollection object. bool insert(int val) Inserts an item val into the multiset, even if the item is already present. Returns true if the item is not present, false otherwise. bool remove(int val) Removes an item val from the multiset if present. Returns true if the item is present, false otherwise. Note that if val has multiple occurrences in the multiset, we only remove one of them. int getRandom() Returns a random element from the current multiset of elements. The probability of each element being returned is linearly related to the number of the same values the multiset contains. You must implement the functions of the class such that each function works on average O(1) time complexity. Note: The test cases are generated such that getRandom will only be called if there is at least one item in the RandomizedCollection. Example 1: Input ["RandomizedCollection", "insert", "insert", "insert", "getRandom", "remove", "getRandom"] [[], [1], [1], [2], [], [1], []] Output [null, true, false, true, 2, true, 1] Explanation RandomizedCollection randomizedCollection = new RandomizedCollection(); randomizedCollection.insert(1); // return true since the collection does not contain 1. // Inserts 1 into the collection. randomizedCollection.insert(1); // return false since the collection contains 1. // Inserts another 1 into the collection. Collection now contains [1,1]. randomizedCollection.insert(2); // return true since the collection does not contain 2. // Inserts 2 into the collection. Collection now contains [1,1,2]. randomizedCollection.getRandom(); // getRandom should: // - return 1 with probability 2/3, or // - return 2 with probability 1/3. randomizedCollection.remove(1); // return true since the collection contains 1. // Removes 1 from the collection. Collection now contains [1,2]. randomizedCollection.getRandom(); // getRandom should return 1 or 2, both equally likely. Constraints: -231 <= val <= 231 - 1 At most 2 * 105 calls in total will be made to insert, remove, and getRandom. There will be at least one element in the data structure when getRandom is called.

Topics: Array, Hash Table, Math, Design, Randomized

Problem #6: String Compression

Given an array of characters chars, compress it using the following algorithm: Begin with an empty string s. For each group of consecutive repeating characters in chars: If the group's length is 1, append the character to s. Otherwise, append the character followed by the group's length. The compressed string s should not be returned separately, but instead, be stored in the input character array chars. Note that group lengths that are 10 or longer will be split into multiple characters in chars. After you are done modifying the input array, return the new length of the array. You must write an algorithm that uses only constant extra space. Example 1: Input: chars = ["a","a","b","b","c","c","c"] Output: Return 6, and the first 6 characters of the input array should be: ["a","2","b","2","c","3"] Explanation: The groups are "aa", "bb", and "ccc". This compresses to "a2b2c3". Example 2: Input: chars = ["a"] Output: Return 1, and the first character of the input array should be: ["a"] Explanation: The only group is "a", which remains uncompressed since it's a single character. Example 3: Input: chars = ["a","b","b","b","b","b","b","b","b","b","b","b","b"] Output: Return 4, and the first 4 characters of the input array should be: ["a","b","1","2"]. Explanation: The groups are "a" and "bbbbbbbbbbbb". This compresses to "ab12". Constraints: 1 <= chars.length <= 2000 chars[i] is a lowercase English letter, uppercase English letter, digit, or symbol.

Topics: Two Pointers, String

Problem #7: Coin Change

You are given an integer array coins representing coins of different denominations and an integer amount representing a total amount of money. Return the fewest number of coins that you need to make up that amount. If that amount of money cannot be made up by any combination of the coins, return -1. You may assume that you have an infinite number of each kind of coin. Example 1: Input: coins = [1,2,5], amount = 11 Output: 3 Explanation: 11 = 5 + 5 + 1 Example 2: Input: coins = [2], amount = 3 Output: -1 Example 3: Input: coins = [1], amount = 0 Output: 0 Constraints: 1 <= coins.length <= 12 1 <= coins[i] <= 231 - 1 0 <= amount <= 104

Topics: Array, Dynamic Programming, Breadth-First Search