![https://leetcode.com/problems/number-of-provinces/] There are n cities. Some of them are connected, while some are not. If city a is connected directly with city b, and city b is connected directly with city c, then city a is connected indirectly with city c. A province is a group of directly or indirectly connected cities and no other cities outside of the group. You are given an n x n matrix isConnected where isConnected[i][j] = 1 if the ith city and the jth city are directly connected, and isConnected[i][j] = 0 otherwise....

![https://leetcode.com/problems/redundant-connection/] In this problem, a tree is an undirected graph that is connected and has no cycles. The given input is a graph that started as a tree with N nodes (with distinct values 1, 2, …, N), with one additional edge added. The added edge has two different vertices chosen from 1 to N, and was not an edge that already existed. The resulting graph is given as a 2D-array of edges....

You are given a list of songs where the ith song has a duration of time[i] seconds. Return the number of pairs of songs for which their total duration in seconds is divisible by 60. Formally, we want the number of indices i, j such that i < j with (time[i] + time[j]) % 60 == 0. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Example 1: Input: time = [30,20,150,100,40] Output: 3 Explanation: Three pairs have a total duration divisible by 60: (time[0] = 30, time[2] = 150): total duration 180 (time[1] = 20, time[3] = 100): total duration 120 (time[1] = 20, time[4] = 40): total duration 60 Example 2: Input: time = [60,60,60] Output: 3 Explanation: All three pairs have a total duration of 120, which is divisible by 60....

Given a binary tree, find the lowest common ancestor (LCA) of two given nodes in the tree. According to the definition of LCA on Wikipedia: “The lowest common ancestor is defined between two nodes p and q as the lowest node in T that has both p and q as descendants (where we allow a node to be a descendant of itself).” 1 2 3 4 5 Example 1: Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1 Output: 3 Explanation: The LCA of nodes 5 and 1 is 3....

An a N x N grid composed of 1 x 1 squares, each 1 x 1 square consists of a /, , or blank space. These characters divide the square into contiguous regions. (Note that backslash characters are escaped, so a \ is represented as “\”.) Return the number of regions. Example 1: 1 2 3 4 5 6 7 Input: [ " /", "/ " ] Output: 2 Explanation: The 2x2 grid is as follows: Example 2:...

Given a string s, find the length of the longest substring without repeating characters. Example 1: 1 2 3 Input: s = "abcabcbb" Output: 3 Explanation: The answer is "abc", with the length of 3. Example 2: 1 2 3 Input: s = "bbbbb" Output: 1 Explanation: The answer is "b", with the length of 1. Example 3: 1 2 3 4 Input: s = "pwwkew" Output: 3 Explanation: The answer is "wke", with the length of 3....

Given two strings A and B of lowercase letters, return true if you can swap two letters in A so the result is equal to B, 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 A[i] and A[j]. For example, swapping at indices 0 and 2 in “abcd” results in “cbad”. Example 1: Input: A = “ab”, B = “ba” Output: true Explanation: You can swap A[0] = ‘a’ and A[1] = ‘b’ to get “ba”, which is equal to B....

Implement int sqrt(int x). Compute and return the square root of x, where x is guaranteed to be a non-negative integer. Since the return type is an integer, the decimal digits are truncated and only the integer part of the result is returned. Example 1: 1 2 Input: 4 Output: 2 Example 2: 1 2 Input: 8 Output: 2 Explanation: The square root of 8 is 2.82842…, and since the decimal part is truncated, 2 is returned....

Implement pow(x, n), which calculates x raised to the power n (i.e. xn). Example 1: Input: x = 2.00000, n = 10 Output: 1024.00000 Example 2: Input: x = 2.10000, n = 3 Output: 9.26100 Example 3: Input: x = 2.00000, n = -2 Output: 0.25000 Explanation: 2-2 = 1/22 = 1/4 = 0.25 Constraints: -100.0 < x < 100.0 -231 <= n <= 231-1 -104 <= xn <= 104...

Given a binary tree, return the zigzag level order traversal of its nodes’ values. (ie, from left to right, then right to left for the next level and alternate between). For example: Given binary tree [3,9,20,null,null,15,7], 1 2 3 4 5 3 / \ 9 20 / \ 15 7 return its zigzag level order traversal as: 1 2 3 4 5 [ [3], [20,9], [15,7] ] Solution: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 /** * Definition for a binary tree node....

Given a non-empty array of integers, return the k most frequent elements. Example 1: Input: nums = [1,1,1,2,2,3], k = 2 Output: [1,2] Example 2: Input: nums = [1], k = 1 Output: [1] Note: You may assume k is always valid, 1 ≤ k ≤ number of unique elements. Your algorithm’s time complexity must be better than O(n log n), where n is the array’s size. It’s guaranteed that the answer is unique, in other words the set of the top k frequent elements is unique....

Given an integer array, find three numbers whose product is maximum and output the maximum product. Example 1: 1 2 Input: [1,2,3] Output: 6 Example 2: 1 2 Input: [1,2,3,4] Output: 24 Note: The length of the given array will be in range [3,104] and all elements are in the range [-1000, 1000]. Multiplication of any three numbers in the input won’t exceed the range of 32-bit signed integer. Solution:...

A company has n employees with a unique ID for each employee from 0 to n - 1. The head of the company has is the one with headID. Each employee has one direct manager given in the manager array where manager[i] is the direct manager of the i-th employee, manager[headID] = -1. Also it’s guaranteed that the subordination relationships have a tree structure. The head of the company wants to inform all the employees of the company of an urgent piece of news....

For a binary tree T, we can define a flip operation as follows: choose any node, and swap the left and right child subtrees. A binary tree X is flip equivalent to a binary tree Y if and only if we can make X equal to Y after some number of flip operations. Given the roots of two binary trees root1 and root2, return true if the two trees are flip equivelent or false otherwise....

Roman numerals are represented by seven different symbols: I, V, X, L, C, D and M. 1 2 3 4 5 6 7 8 Symbol Value I 1 V 5 X 10 L 50 C 100 D 500 M 1000 For example, two is written as II in Roman numeral, just two one’s added together. Twelve is written as, XII, which is simply X + II. The number twenty seven is written as XXVII, which is XX + V + II....