1254_number_of_closed_islands.cpp

#include<bits/stdc++.h>
using namespace std;

/*
1254. Number of Closed Islands
Medium
3.8K
128
Companies
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.


NO IDEA:) except BFS and DFS
*/
class Solution {
public:
    int closedIsland(vector<vector<int>>& grid) {
        int m = grid.size();
        int n = grid[0].size();
        int count = 0;
        vector<vector<bool>> visit(m, vector<bool>(n));
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (grid[i][j] == 0 && !visit[i][j] && dfs(i, j, m, n, grid, visit)) {
                    count++;
                }
            }
        }
        return count;
    }

    bool dfs(int x, int y, int m, int n, vector<vector<int>>& grid, vector<vector<bool>>& visit) {
        if (x < 0 || x >= m || y < 0 || y >= n) {
            return false;
        }
        if (grid[x][y] == 1 || visit[x][y]) {
            return true;
        }

        visit[x][y] = true;
        bool isClosed = true;
        vector<int> dirx {0, 1, 0, -1};
        vector<int> diry {-1, 0, 1, 0};

        for (int i = 0; i < 4; i++) {
            int r = x + dirx[i];
            int c = y + diry[i];
            if (!dfs(r, c, m, n, grid, visit)) {
                isClosed = false;
            }
        }

        return isClosed;
    }
};