The Easiest Way to Solve LeetCode 3619 (Count Islands Divisible by K)

Graph problems don’t have to be terrifying. In this video, we tackle LeetCode 3619: Count Islands With Total Value Divisible by K. This is a fantastic Medium-level problem that perfectly tests your ability to navigate 2D matrices using Depth-First Search (DFS) or Breadth-First Search (BFS) to find connected components. Whether you are grinding for your next FAANG interview or just want to level up your graph traversal skills, I'll break down the intuition step-by-step so you actually understand the math and the code behind it. We will cover how to efficiently group our "land" cells, sum their values, and check for divisibility without timing out. What you will learn in this video: How to identify connected components in a 2D grid. When to use DFS vs. BFS for matrix traversal. Handling modulo arithmetic efficiently in competitive programming. Time and space complexity analysis for our optimal solution. Timestamps: 0:00 - The Problem Breakdown 2:15 - The Core Logic: Why DFS/BFS? 4:30 - Whiteboard Explanation & Edge Cases 7:10 - Writing the Code (Optimal Solution) 12:00 - Complexity Analysis & Final Thoughts Drop a comment below with your preferred method—are you Team DFS or Team BFS? Let me know!