Introduction

In this blog post, we'll share the most commonly asked coding interview questions at Snowflake. 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: String Transformation

You are given two strings s and t of equal length n. You can perform the following operation on the string s: Remove a suffix of s of length l where 0 < l < n and append it at the start of s. For example, let s = 'abcd' then in one operation you can remove the suffix 'cd' and append it in front of s making s = 'cdab'. You are also given an integer k. Return the number of ways in which s can be transformed into t in exactly k operations. Since the answer can be large, return it modulo 109 + 7. Example 1: Input: s = "abcd", t = "cdab", k = 2 Output: 2 Explanation: First way: In first operation, choose suffix from index = 3, so resulting s = "dabc". In second operation, choose suffix from index = 3, so resulting s = "cdab". Second way: In first operation, choose suffix from index = 1, so resulting s = "bcda". In second operation, choose suffix from index = 1, so resulting s = "cdab". Example 2: Input: s = "ababab", t = "ababab", k = 1 Output: 2 Explanation: First way: Choose suffix from index = 2, so resulting s = "ababab". Second way: Choose suffix from index = 4, so resulting s = "ababab". Constraints: 2 <= s.length <= 5 * 105 1 <= k <= 1015 s.length == t.length s and t consist of only lowercase English alphabets.

Topics: Math, String, Dynamic Programming, String Matching

Problem #2: Happy Number

Write an algorithm to determine if a number n is happy. A happy number is a number defined by the following process: Starting with any positive integer, replace the number by the sum of the squares of its digits. Repeat the process until the number equals 1 (where it will stay), or it loops endlessly in a cycle which does not include 1. Those numbers for which this process ends in 1 are happy. Return true if n is a happy number, and false if not. Example 1: Input: n = 19 Output: true Explanation: 12 + 92 = 82 82 + 22 = 68 62 + 82 = 100 12 + 02 + 02 = 1 Example 2: Input: n = 2 Output: false Constraints: 1 <= n <= 231 - 1

Topics: Hash Table, Math, Two Pointers

Problem #3: Trapping Rain Water

Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it can trap after raining. Example 1: Input: height = [0,1,0,2,1,0,1,3,2,1,2,1] Output: 6 Explanation: The above elevation map (black section) is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped. Example 2: Input: height = [4,2,0,3,2,5] Output: 9 Constraints: n == height.length 1 <= n <= 2 * 104 0 <= height[i] <= 105

Topics: Array, Two Pointers, Dynamic Programming, Stack, Monotonic Stack

Problem #4: Number of Ways to Form a Target String Given a Dictionary

You are given a list of strings of the same length words and a string target. Your task is to form target using the given words under the following rules: target should be formed from left to right. To form the ith character (0-indexed) of target, you can choose the kth character of the jth string in words if target[i] = words[j][k]. Once you use the kth character of the jth string of words, you can no longer use the xth character of any string in words where x <= k. In other words, all characters to the left of or at index k become unusuable for every string. Repeat the process until you form the string target. Notice that you can use multiple characters from the same string in words provided the conditions above are met. Return the number of ways to form target from words. Since the answer may be too large, return it modulo 109 + 7. Example 1: Input: words = ["acca","bbbb","caca"], target = "aba" Output: 6 Explanation: There are 6 ways to form target. "aba" -> index 0 ("acca"), index 1 ("bbbb"), index 3 ("caca") "aba" -> index 0 ("acca"), index 2 ("bbbb"), index 3 ("caca") "aba" -> index 0 ("acca"), index 1 ("bbbb"), index 3 ("acca") "aba" -> index 0 ("acca"), index 2 ("bbbb"), index 3 ("acca") "aba" -> index 1 ("caca"), index 2 ("bbbb"), index 3 ("acca") "aba" -> index 1 ("caca"), index 2 ("bbbb"), index 3 ("caca") Example 2: Input: words = ["abba","baab"], target = "bab" Output: 4 Explanation: There are 4 ways to form target. "bab" -> index 0 ("baab"), index 1 ("baab"), index 2 ("abba") "bab" -> index 0 ("baab"), index 1 ("baab"), index 3 ("baab") "bab" -> index 0 ("baab"), index 2 ("baab"), index 3 ("baab") "bab" -> index 1 ("abba"), index 2 ("baab"), index 3 ("baab") Constraints: 1 <= words.length <= 1000 1 <= words[i].length <= 1000 All strings in words have the same length. 1 <= target.length <= 1000 words[i] and target contain only lowercase English letters.

Topics: Array, String, Dynamic Programming

Problem #5: Painting the Walls

You are given two 0-indexed integer arrays, cost and time, of size n representing the costs and the time taken to paint n different walls respectively. There are two painters available: A paid painter that paints the ith wall in time[i] units of time and takes cost[i] units of money. A free painter that paints any wall in 1 unit of time at a cost of 0. But the free painter can only be used if the paid painter is already occupied. Return the minimum amount of money required to paint the n walls. Example 1: Input: cost = [1,2,3,2], time = [1,2,3,2] Output: 3 Explanation: The walls at index 0 and 1 will be painted by the paid painter, and it will take 3 units of time; meanwhile, the free painter will paint the walls at index 2 and 3, free of cost in 2 units of time. Thus, the total cost is 1 + 2 = 3. Example 2: Input: cost = [2,3,4,2], time = [1,1,1,1] Output: 4 Explanation: The walls at index 0 and 3 will be painted by the paid painter, and it will take 2 units of time; meanwhile, the free painter will paint the walls at index 1 and 2, free of cost in 2 units of time. Thus, the total cost is 2 + 2 = 4. Constraints: 1 <= cost.length <= 500 cost.length == time.length 1 <= cost[i] <= 106 1 <= time[i] <= 500

Topics: Array, Dynamic Programming

Problem #6: Count Vowel Substrings of a String

A substring is a contiguous (non-empty) sequence of characters within a string. A vowel substring is a substring that only consists of vowels ('a', 'e', 'i', 'o', and 'u') and has all five vowels present in it. Given a string word, return the number of vowel substrings in word. Example 1: Input: word = "aeiouu" Output: 2 Explanation: The vowel substrings of word are as follows (underlined): - "aeiouu" - "aeiouu" Example 2: Input: word = "unicornarihan" Output: 0 Explanation: Not all 5 vowels are present, so there are no vowel substrings. Example 3: Input: word = "cuaieuouac" Output: 7 Explanation: The vowel substrings of word are as follows (underlined): - "cuaieuouac" - "cuaieuouac" - "cuaieuouac" - "cuaieuouac" - "cuaieuouac" - "cuaieuouac" - "cuaieuouac" Constraints: 1 <= word.length <= 100 word consists of lowercase English letters only.

Topics: Hash Table, String

Problem #7: 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 #9: Course Schedule II

There are a total of numCourses courses you have to take, labeled from 0 to numCourses - 1. You are given an array prerequisites where prerequisites[i] = [ai, bi] indicates that you must take course bi first if you want to take course ai. For example, the pair [0, 1], indicates that to take course 0 you have to first take course 1. Return the ordering of courses you should take to finish all courses. If there are many valid answers, return any of them. If it is impossible to finish all courses, return an empty array. Example 1: Input: numCourses = 2, prerequisites = [[1,0]] Output: [0,1] Explanation: There are a total of 2 courses to take. To take course 1 you should have finished course 0. So the correct course order is [0,1]. Example 2: Input: numCourses = 4, prerequisites = [[1,0],[2,0],[3,1],[3,2]] Output: [0,2,1,3] Explanation: There are a total of 4 courses to take. To take course 3 you should have finished both courses 1 and 2. Both courses 1 and 2 should be taken after you finished course 0. So one correct course order is [0,1,2,3]. Another correct ordering is [0,2,1,3]. Example 3: Input: numCourses = 1, prerequisites = [] Output: [0] Constraints: 1 <= numCourses <= 2000 0 <= prerequisites.length <= numCourses * (numCourses - 1) prerequisites[i].length == 2 0 <= ai, bi < numCourses ai != bi All the pairs [ai, bi] are distinct.

Topics: Depth-First Search, Breadth-First Search, Graph, Topological Sort

Problem #10: 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. If there is no such substring, return the empty string "". The testcases will be generated such that the answer is unique. Example 1: Input: s = "ADOBECODEBANC", t = "ABC" Output: "BANC" Explanation: The minimum window substring "BANC" includes 'A', 'B', and 'C' from string t. Example 2: Input: s = "a", t = "a" Output: "a" Explanation: The entire string s is the minimum window. Example 3: Input: s = "a", t = "aa" Output: "" Explanation: Both 'a's from t must be included in the window. Since the largest window of s only has one 'a', return empty string. Constraints: m == s.length n == t.length 1 <= m, n <= 105 s and t consist of uppercase and lowercase English letters. Follow up: Could you find an algorithm that runs in O(m + n) time?

Topics: Hash Table, String, Sliding Window