Number of IslandsSolution

Description

Given a 2d grid map of '1's (land) and '0's (water), count the number of islands. An island is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.

Example 1:

Input:

11110

11010

11000

00000

Output: 1

Example 2:

Input:

11000

11000

00100

00011

Output: 3

Code

class Solution {
    
    int m;
    int n;
    
    public int numIslands(char[][] grid) {
        m = grid.length;
        if (m == 0) return 0;
        n = grid[0].length;
        int count = 0;
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (grid[i][j] == '1') {
                    DFSMarking(grid, i, j);
                    ++count;
                }
            }
        }
        return count;
    }
    
    public void DFSMarking(char[][] grid, int i, int j) {
        if (i < 0 || i >= m || j < 0 || j >= n || grid[i][j] == '0') return;
        grid[i][j] = '0';
        DFSMarking(grid, i + 1, j);
        DFSMarking(grid, i - 1, j);
        DFSMarking(grid, i, j + 1);
        DFSMarking(grid, i, j - 1);
    }
}