Given an array nums with n integers, your task is to check if it could become non-decreasing by modifying at most one element.We define an array as non-decreasing if nums[i] <= nums[i+1] holds for every i (0-based) such that 0 <= i <= n-2. English Summary (Approach)Single pass. Count violations where nums[i] > nums[i+1]. If…

Find the unique element in an array where all elements appear twice except one.Examples: Summary (with approach)Use XOR: since a ^ a = 0, a ^ 0 = a, and XOR is associative/commutative, XOR all numbers; the result is the single number. Runs in O(N) time and O(1) space. Alternative: hash counting in O(N) space….

Imagine an array that contains both integers and nested arrays, such as the following:[8, 4, [5, [9], 3], 6].The depth sum is described as the weighted sum of each integer, weighted by their respective depths. In the example, 8’s depth is 1, while 9’s is 3.Given such an array, calculate its depth sum. Examples Summary…

You are given K sorted integer arrays (non-decreasing). Arrays may contain duplicates within the same array and across different arrays.Return a sorted list of unique integers that appear in any of the arrays (i.e., the set union, sorted). Example:A = [-100, -1, -1, 0, 5]B = [0]C = [-1, 0, 0]Output → [-100, -1, 0,…

Given the root of a binary search tree, return a balanced binary search tree that contains the same node values. If there is more than one valid answer, return any of them.A binary search tree is balanced if the depth of the two subtrees of every node never differs by more than 1. Example:Input: root…

Problem (English original):Given the root of a binary tree containing integers, return a list of all the integers in the tree ordered by column from left to right. Within each column, values should be ordered from top to bottom. Assume a node definition (or language equivalent): Input (example, ASCII tree): Output (example): Summary (approach):BFS/DFS tagging…

Given a string S consisting of lowercase English characters, determine if you can make it a palindrome by removing at most one character. Example: Summary (approach):Use two pointers. On the first mismatch, try skipping either the left or the right character and check if the remaining substring is a palindrome. O(n) time, O(1) space. The VOprep team…

Problem (verbatim):“A multiset (also known as a bag) is a mutable, unordered collection of distinct objects that may appear more than once in the collection. Implement a multiset that implements the following methods:• add(element)• remove(element)• count(for: element)” English summary (concise approach):Back the multiset with a hash map value → frequency. The VOprep team has long accompanied candidates…

Problem (verbatim):“Given an integer array and an integer number k. Return the k-th largest element in the array. Examples:• array = [5, −3, 9, −1]• k = 0 ⇒ return: 9• k = 1 ⇒ return: 5• k = 3 ⇒ return: −3” summary (concise approach):Return the k-th largest (0-indexed) value. The VOprep team has long accompanied…

Problem Statement (English original)Assume a binary-tree node: Given the root of a binary tree, output the node values in vertical order (column order) from leftmost column to rightmost column.Within each column, list nodes top-to-bottom in the order they are encountered by a standard BFS (or by row index if you compute coordinates). Illustrative example (matching…

Problem Statement (English original)You are given a current working directory cwd (an absolute Unix-style path) and a cd(arg) string which can be either an absolute path (starts with /) or a relative path (may include segments . and ..).Implement a function that returns the normalized absolute path after applying cd(arg) from cwd. Rules: Examples (from…

You are given a 2-D grid representing two elevators (left elevator A, right elevator B).Each cell may contain a digit representing the number of people waiting on that floor for that elevator. Example grid: Legend: You are also given a query describing a request: Task Given the grid and a request in the format: Write…

You are given a list of users, each with a start time and an end time.For each time interval where the set of active users changes, output: Input Example Output Example Convert all start/end into events: Sort events and sweep from left to right.Maintain an active user set: Between consecutive event times, output a time…

Given a log file, find the top N most talkative users Parse each line, extract username and message, count words, aggregate per user in a map, sort by total word count (desc), return top N. Given a chat log, count how many words each user has spoken and return the top N most talkative users….

You have a list of meetings in your calendar with a start and end time. You are very busy, so meetings can overlap. You also have one “Do Not Schedule”(DNS) interval during which you don’t attend any meeting. Any meeting schedule that overlaps with a DNS slot is automatically cut such that it does not…

📌 Part A — Tree Islands Problem Question:In a tree of zeros and ones, an island is defined as a group of ones that are surrounded by zeros or are at the boundary of the tree. Example: Find the number of islands in the tree. In the above example, there are 4 islands. 📌 Part…

This task requires you to sort characters in a string by: ✅ Rules ✅ Approach This tests basic algorithm design and comfort with the language, without relying heavily on advanced libraries. The VOprep team has long accompanied candidates through various major company OAs and VOs, including OpenAI, Google, Amazon, Citadel, SIG, providing real-time voice assistance, remote practice,…

Warmup Section Imagine you have a robot that sends status messages that humans will read in real time.The raw messages are hard to read for a human because there are often many messages produced in short periods of time. One idea to make them more readable is to remove the duplicate messages over a 10-second…

Given an English-language text and the width of a page, find how many lines the text would take on that page. Given a text that you’d like to place into a two-column table on a fixed-width page, find the best column width that minimizes the overall table height. Subproblem 1 (line count): Given page width,…

Here we have an event log file produced by a Friends service, e.g.: Given a list of all users and the logs above, implement a function that returns the earliest timestamp at which every user became reachable from every other user via friendship connections (i.e., when the social graph first became fully connected).If the graph…