11110
11010
11000
00000

11000
11000
00100
00011

public class Solution { private int m, n; public int numIslands(char[][] grid) { m = grid.length; if (m == 0) return 0; n = grid[0].length; if (n == 0) return 0; int ans = 0; for (int i = 0; i < m; i++) { for (int j = 0; j < n; j++) { if (grid[i][j] != '1') continue; ans++; dfs(grid, i, j); } } return ans; } public void dfs(char[][] grid, int i, int j) { if (i < 0 || i >= m || j < 0 || j >= n) return; if (grid[i][j] == '1') { grid[i][j] = '2'; dfs(grid, i - 1, j); dfs(grid, i + 1, j); dfs(grid, i, j - 1); dfs(grid, i, j + 1); } } }