Number of Islands in Java: Explanation & Practice

Count number of islands in a 2D grid

Problem summary

Given a 2D grid of '1's (land) and '0's (water), count the number of islands. An island is surrounded by water and formed by connecting adjacent lands horizontally or vertically.

Starter code

public class Main {
    public static void main(String[] args) {
        char[][] grid = {
            {'1','1','0','0','0'},
            {'1','1','0','0','0'},
            {'0','0','1','0','0'},
            {'0','0','0','1','1'}
        };
        
        // DFS/BFS from each unvisited '1'
        // Mark all connected land as visited
        // Count number of DFS/BFS calls
        // Print: Islands: 3
    }
}

Expected output and test cases

3 separate islands
```
Islands: 3
```
All connected → 1 island
```
Islands: 1
```
All water → 0
```
Islands: 0
```

Hints

Iterate through grid, when you find '1', start DFS/BFS
Mark visited cells by changing '1' to '0' or using visited array
DFS explores all 4 directions: up, down, left, right
Each DFS/BFS from a new '1' = one new island

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