How do you solve Number of Islands?

Short answer: Scan the grid. When you find a '1', increment island count and DFS/BFS to sink (mark visited) the entire connected land component. Count how many times you start a flood fill.

Complexity

Time O(rows * cols), Space O(rows * cols) worst-case recursion/queue.

Edge cases to mention

• Empty grid
• All water
• Diagonal connectivity? (usually 4-directional only)

Common follow-ups

• Max area of island
• Number of distinct islands
• Surrounded regions

Clarify 4-direction vs 8-direction before coding.