200 Number of Islands

感受DFS

private int m;
    private int n;

    public int numIslands(char[][] grid) {
        int res = 0;
        m = grid.length;
        if (m == 0) return 0;
        n = grid[0].length;
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (grid[i][j] == '1') {
                    dfs(grid, i, j);
                    res++;
                }
            }
        }
        return res;
    }

    /**
     time : O(m * n)
     space : O(m * n)
     * @param grid
     * @param i
     * @param j
     */

    private void dfs(char[][] grid, int i, int j) {
        if (i < 0 || j < 0 || i >= m || j >= n || grid[i][j] == '0') return;
        grid[i][j] = '0';
        dfs(grid, i, j + 1);
        dfs(grid, i, j - 1);
        dfs(grid, i + 1, j);
        dfs(grid, i - 1, j);
    }