<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:
    int closedIsland(vector<vector<int>>& grid) 
    {
        int m = grid.size(), n = grid[0].size(), i, j, cnt = 0;
        vector<vector<bool>> vis(m, vector<bool>(n, false));

        for(i = 0; i < m; i++)
        {
            for(j = 0; j < n; j++)
            {
                if(0 == grid[i][j] && !vis[i][j] && helper(i, j, grid, vis))
                {
                    cnt++;
                }
            }
        }

        return cnt;
    }

private:
    bool helper(int x, int y, vector<vector<int>> &grid, vector<vector<bool>> &vis)
    {
        int m = grid.size(), n = grid[0].size(), i, j;
        vector<pair<int, int>> dirs = {{-1, 0}, {1, 0}, {0, -1}, {0, 1}};
        bool res = true;

        if(x < 0 || x >= m || y < 0 || y >= n)
        {
            return false;
        }
        else if(1 == grid[x][y] || vis[x][y])
        {
            return true;
        }

        vis[x][y] = true;

        for(auto &d: dirs)
        {
            i = x + d.first;
            j = y + d.second;

            if(!helper(i, j, grid, vis))
            {
                res = false;
            }
        }

        return res;
    }
};
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