We solved two LeetCode problems with DFS and BFS algorithms in this story: Two examples of DFS/BFS graph traversal The input matrices are a connectivity matrix and an island matrix, respectively. Example 1: 547. Number of Provinces Each cell in the matrix represents an edge. Given the condition, then x n matrix isConnected where isConnected[i][j] =…

The two LeetCode problems that we are going to discuss are similar because they both can be solved by DFS or BFS algorithms, but different because the input matrices are a connectivity matrix vs. an island matrix. Example 1: 547. Number of Provinces Each cell in the matrix represents an…

907. Sum of Subarray Minimums Given an array of integers arr, find the sum of min(b), where b ranges over every (contiguous) subarray of arr. Since the answer may be large, return the answer modulo 109 + 7. One pattern to utilize is the number of subarrays for each element…

Many string problems involve the check and count of valid parenthesis. 2116. Check if a Parentheses String Can Be Valid 32. Longest Valid Parentheses Parenthesis are balanced only if it starts with ( and ends with ). Here are some properties of a valid parentheses string: The length of a…

Today we are going to solve a pretty straightforward yet a bit interesting problem. LeetCode 2216 You are given a 0-indexed integer array nums. The array nums is beautiful if: nums.length is even. nums[i] != nums[i + 1] for all i % 2 == 0. Note that an empty array…

Greedy problems usually look like “Find the minimum number of something to do something” or “Find the maximum number of something to fit in some conditions”, and typically propose an unsorted input. Here is an example: A company is planning to interview 2n people. Given the array costs where costs[i]…

Prefix sum is a very common method to solve many array problems. Here is an interesting one: Given an integer length and an array updates where updates[i] = [startIdxi, endIdxi, inci]. You have an array arr of length length with all zeros, and you have some operation to apply on…

Lowest Common Ancestor is one of the commonest tree problems in code challenges. Let’s dive deep into the series of problems and try to conclude some simple essence that could help us solve similar problems. Problem family: Lowest Common Ancestor of a Binary Search Tree Lowest Common Ancestor of a…