<p>Given a 2D&nbsp;<code>grid</code> consists of <code>0s</code> (land)&nbsp;and <code>1s</code> (water).&nbsp; An <em>island</em> is a maximal 4-directionally connected group of <code><font face="monospace">0</font>s</code> and a <em>closed island</em>&nbsp;is an island <strong>totally</strong>&nbsp;(all left, top, right, bottom) surrounded by <code>1s.</code></p>

<p>Return the number of <em>closed islands</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>

<p><img alt="" src="https://assets.leetcode.com/uploads/2019/10/31/sample_3_1610.png" style="width: 240px; height: 120px;" /></p>

<pre>
<strong>Input:</strong> 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]]
<strong>Output:</strong> 2
<strong>Explanation:</strong> 
Islands in gray are closed because they are completely surrounded by water (group of 1s).</pre>

<p><strong class="example">Example 2:</strong></p>

<p><img alt="" src="https://assets.leetcode.com/uploads/2019/10/31/sample_4_1610.png" style="width: 160px; height: 80px;" /></p>

<pre>
<strong>Input:</strong> grid = [[0,0,1,0,0],[0,1,0,1,0],[0,1,1,1,0]]
<strong>Output:</strong> 1
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> grid = [[1,1,1,1,1,1,1],
&nbsp;              [1,0,0,0,0,0,1],
&nbsp;              [1,0,1,1,1,0,1],
&nbsp;              [1,0,1,0,1,0,1],
&nbsp;              [1,0,1,1,1,0,1],
&nbsp;              [1,0,0,0,0,0,1],
               [1,1,1,1,1,1,1]]
<strong>Output:</strong> 2
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>1 &lt;= grid.length, grid[0].length &lt;= 100</code></li>
	<li><code>0 &lt;= grid[i][j] &lt;=1</code></li>
</ul>

class Solution {
public:
    bool checkBoundaryCell(int x, int y, int m, int n) {
        return (x == 0 || x == m - 1 || y == 0 || y == n - 1);
    }
    int closedIsland(vector<vector<int>>& grid) {
        int m = grid.size();
        int n = grid[0].size();
        int cnt = 0;
        vector<vector<int>> moves={{-1, 0}, {0, 1}, {1, 0}, {0, -1}};
        for(int i = 0; i < m; i++) {
            for(int j = 0; j < n; j++) {
                if(grid[i][j] == 1) continue;
                queue<pair<int, int>> q;
                q.push({i, j});
                bool isIslandClosed = true;
                grid[i][j] = 1;
                while(!q.empty()) {
                    pair<int, int> p = q.front();
                    int x = p.first;
                    int y = p.second;
                    q.pop();
                    if(checkBoundaryCell(x, y, m, n)) {
                        isIslandClosed = false;
                    }
                    for(auto move : moves) {
                        int dx = x + move[0];
                        int dy = y + move[1];

                        if(dx >= 0 and dy >= 0 and dx < m and dy < n
                            and grid[dx][dy] == 0) {
                            q.push({dx, dy});
                            grid[dx][dy] = 1;
                        }
                    }
                }

                cnt += isIslandClosed;
            }
        }
        return cnt;
    }
};
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