题目：

题解：

class Solution {
private:void dfs(vector<vector<char>>& grid, int r, int c) {int nr = grid.size();int nc = grid[0].size();grid[r][c] = '0';if (r - 1 >= 0 && grid[r-1][c] == '1') dfs(grid, r - 1, c);if (r + 1 < nr && grid[r+1][c] == '1') dfs(grid, r + 1, c);if (c - 1 >= 0 && grid[r][c-1] == '1') dfs(grid, r, c - 1);if (c + 1 < nc && grid[r][c+1] == '1') dfs(grid, r, c + 1);}public:int numIslands(vector<vector<char>>& grid) {int nr = grid.size();if (!nr) return 0;int nc = grid[0].size();int num_islands = 0;for (int r = 0; r < nr; ++r) {for (int c = 0; c < nc; ++c) {if (grid[r][c] == '1') {++num_islands;dfs(grid, r, c);}}}return num_islands;}
};