1254. Number of Closed Islands.cpp

class Solution {
public:
    bool isValid(int i, int j, int n, int m, vector<vector<int>>& grid){
        if(i>=0 and j>=0 and i<n and j<m and grid[i][j]==0){
            return true;
        }
        return false;
    }
    
    void dfs(int i, int j, int n, int m, vector<vector<int>>& grid){
        grid[i][j] = 1;
        
        int ax[4] = {1, -1, 0, 0};
        int ay[4] = {0, 0, 1, -1};
        
        for(int k=0; k<4; k++){
            int nx = i + ax[k];
            int ny = j + ay[k];
            if(isValid(nx, ny, n, m, grid)){
                dfs(nx, ny, n, m, grid);
            }
        }
    }
    
    int closedIsland(vector<vector<int>>& grid) {
        int n = grid.size();
        int m = grid[0].size();
        
        //boundary DFS (changing 0 to 1). the 0s which will be left will be the ones who are completely locked by land
        for(int i=0; i<n; i++){
            for(int j=0; j<m; j++){
                if(i*j==0 || i==n-1 || j==m-1){
                    if(grid[i][j] == 0){
                        dfs(i, j, n, m, grid);
                    }
                }
            }
        }
        
        //DFS again for counting closed islands
        int ans = 0;
        for(int i=0; i<n; i++){
            for(int j=0; j<m; j++){
                if(grid[i][j] == 0){
                    ans++;
                    dfs(i, j, n, m, grid);
                } 
            }
        }
        return ans;
    }
};