200. 岛屿数量 - 力扣（LeetCode） (opens new window)

class Solution {
    public int numIslands(char[][] grid) {
        int res = 0;
        int m = grid.length, n = grid[0].length;
        for (int i = 0; i < m ; i++) {
            for (int j = 0; j < n; j++) {
                if (grid[i][j] == '1') {
                    // 每发现⼀个岛屿，岛屿数量加⼀
                    res ++;
                    // 然后使⽤DFS将岛屿淹了
                    dfs(grid, i, j);
                }
            }
        }
        return res;
    }
    // 从(i, j) 开始，将与之相邻的陆地都变成海⽔
    void dfs(char[][] grid, int i, int j) {
        int m = grid.length, n = grid[0].length;
        if (i < 0 || j < 0 || i >= m || j >= n) {
            return;
        }
        if (grid[i][j] == '0') return;
        // 将(i, j) 变成海⽔
        grid[i][j] = '0';
        dfs(grid, i + 1, j);
        dfs(grid, i, j + 1);
        dfs(grid, i - 1, j);
        dfs(grid, i, j - 1);
    }
}