200. 岛屿数量

// LeetCode指定调用方法     
public int numIslands(char[][] grid) {
		
    int result = 0;

    for (int i = 0; i < grid.length; i++) {
        for (int j = 0; j < grid[0].length; j++) {

            // 值为'1'时进行一次DFS
            if (grid[i][j] == '1') {
                dfs(grid, i, j);
                result++;
            }
        }
    }

    return result;
}

// DFS
private void dfs(char[][] grid, int a, int b) {

    // 该位置已超出矩阵限制或该位置值为'0'，结束搜索
    if (a < 0 || b < 0 || a >= grid.length || b >= grid[0].length || grid[a][b] == '0') { 
        return ;
    } 

    // 将已搜索过的'1'赋值为'0'
    grid[a][b] = '0';

    // 递归搜索4个方向
    dfs(grid, a + 1, b);
    dfs(grid, a - 1, b);
    dfs(grid, a, b + 1);
    dfs(grid, a, b - 1);
}