【DFS】200.岛屿数量

题目
直接跳转Top100：200.岛屿数量

法1：DFS

最简单的DFS必须掌握！！！

class Solution {
    public int numIslands(char[][] grid) {
        int m = grid.length, n = grid[0].length, ans = 0;
        if (m == 0 || n == 0) {
            return ans;
        }
        boolean[][] visited = new boolean[m][n];
        for (int i = 0; i < m; ++i) {
            for (int j = 0; j < n; ++j) {
                if (grid[i][j] == '1' && !visited[i][j]) {
                    ++ans;
                    dfs(grid, visited, i, j);
                }
            }
        }

        return ans;
    }

    public void dfs(char[][] grid, boolean[][] visited, int i, int j) {
        if (i >= 0 && i < grid.length 
                && j >= 0 && j < grid[0].length 
                && grid[i][j] == '1' 
                && !visited[i][j]) {
            visited[i][j] = true;
            dfs(grid, visited, i - 1, j);
            dfs(grid, visited, i + 1, j);
            dfs(grid, visited, i, j + 1);
            dfs(grid, visited, i, j - 1);
        }
    }
}