1254. Number of Closed Islands

Given a 2D grid consists of 0s (land) and 1s (water).  An island is a maximal 4-directionally connected group of 0s and a closed island is an island totally (all left, top, right, bottom) surrounded by 1s.

Return the number of closed islands.

Example 1:

Input: grid = [[1,1,1,1,1,1,1,0],[1,0,0,0,0,1,1,0],[1,0,1,0,1,1,1,0],[1,0,0,0,0,1,0,1],[1,1,1,1,1,1,1,0]]
Output: 2
Explanation: 
Islands in gray are closed because they are completely surrounded by water (group of 1s).

Example 2:

Input: grid = [[0,0,1,0,0],[0,1,0,1,0],[0,1,1,1,0]]
Output: 1

Example 3:

Input: grid = [[1,1,1,1,1,1,1],
               [1,0,0,0,0,0,1],
               [1,0,1,1,1,0,1],
               [1,0,1,0,1,0,1],
               [1,0,1,1,1,0,1],
               [1,0,0,0,0,0,1],
               [1,1,1,1,1,1,1]]
Output: 2

Constraints:

• 1 <= grid.length, grid[0].length <= 100
• 0 <= grid[i][j] <=1

class Solution {
    public int closedIsland(int[][] grid) {
        int m = grid.length;
        int n = grid[0].length;        
        int res = 0;
        for(int i = 0; i < m; i++){
            for(int j = 0; j < n; j++){
                if(grid[i][j] == 0 && i == 0 || j == 0 || i == m - 1 || j == n - 1) help(i, j, grid);
            }
        }
        for(int i = 0; i < m; i++){
            for(int j = 0; j < n; j++){
                if(grid[i][j] == 0){
                  res++;
                  help(i, j, grid);  
                }
            }
        }
        return res;
    }
    public void help(int r, int c, int[][] grid){
        if(r > -1 && r < grid.length && c > -1 && c < grid[0].length && grid[r][c] == 0) grid[r][c] = 1;
        else return;
        help(r + 1, c, grid);
        help(r, c + 1, grid);
        help(r - 1, c, grid);
        help(r, c - 1, grid);
    }
}

DFS, 先把边界的0区域全置1，然后再遍历，遇到0就先加1，然后再把该0区域置1，最后得到答案。