Company/Facebook

Problem Every valid email consists of a local name and a domain name, separated by the '@' sign. Besides lowercase letters, the email may contain one or more '.' or '+'. For example, in "[email protected]", "alice" is the local name, and "leetcode.com" is the domain name. If you add periods '.' between some characters in the local name part of an email address, mail sent there will be forwarded to the same address without dots in the local name. Note that this rule does not apply to domain names. ...

Problem You are given two binary trees root1 and root2. Imagine that when you put one of them to cover the other, some nodes of the two trees are overlapped while the others are not. You need to merge the two trees into a new binary tree. The merge rule is that if two nodes overlap, then sum node values up as the new value of the merged node. Otherwise, the NOT null node will be used as the node of the new tree. ...

Problem Given an integer array nums, find three numbers whose product is maximum and return the maximum product. Examples Example 1: Input: nums = [1,2,3] Output: 6 Example 2: Input: nums = [1,2,3,4] Output: 24 ...

Problem You are given an n x n integer matrix board where the cells are labeled from 1 to n^2 in a Boustrophedon style starting from the bottom left of the board (i.e. board[n - 1][0]) and alternating direction each row. You start on square 1 of the board. In each move, starting from square curr, do the following: Choose a destination square next with a label in the range [curr + 1, min(curr + 6, n^2)]. This choice simulates the result of a standard 6-sided die roll: i.e., there are always at most 6 destinations, regardless of the size of the board. If next has a snake or ladder, you must move to the destination of that snake or ladder. Otherwise, you move to next. The game ends when you reach the square n^2. A board square on row r and column c has a snake or ladder if board[r][c] != -1. The destination of that snake or ladder is board[r][c]. Squares 1 and n^2 do not have a snake or ladder. ...

Problem Write a program to check whether a given number is an ugly number. Ugly Number Definition Examples Example 1: Input: n = 6 Output: true Explanation: 6 = 2 × 3 Example 2: Input: n = 1 Output: true Explanation: 1 has no prime factors, therefore all of its prime factors are limited to 2, 3, and 5. ...

Problem Given a stream of elements too large to store in memory, pick a random element from the stream with uniform probability. Solution This is a classic problem known as Reservoir Sampling. The goal is to randomly select an element from a large stream of data with uniform probability, even when the entire stream cannot be stored in memory. Introduction to Problem Reservoir sampling, often referred to as Algorithm R as described by Jeffrey Vitter in Random Sampling with a Reservoir, is a widely used technique in data processing. It allows for the random selection of k samples from a set S containing n items, where n is very large or unknown. Each of the chosen k items forms a “reservoir,” ensuring that every item is selected with an equal probability of 1/n. ...

Problem Reverse a linked list from position m to n. Do it in-place and in one-pass. OR Given the head of a singly linked list and two integers left and right where left <= right, reverse the nodes of the list from position left to position right, and return the reversed list. Examples Example 1: --- title: Input --- graph LR A1[1] --> B2[2]:::reversed --> C3[3]:::reversed --> D4[4]:::reversed --> E5[5] classDef reversed fill:#ffcc00,stroke:#000,stroke-width:2px; ...

Problem Given a string, find the length of the longest substring without repeating characters. Examples Example 1: Input: s = "aababcbb" Output: 3 Explanation: The longest substring without repeating characters is "abc". Example 2: Input: s = "cccc" Output: 1 Explanation: The longest substring without repeating characters is "c". ...