<p>You are given an <code>m x n</code> binary grid <code>grid</code> where <code>1</code> represents land and <code>0</code> represents water. An <strong>island</strong> is a maximal <strong>4-directionally</strong> (horizontal or vertical) connected group of <code>1</code>&#39;s.</p>

<p>The grid is said to be <strong>connected</strong> if we have <strong>exactly one island</strong>, otherwise is said <strong>disconnected</strong>.</p>

<p>In one day, we are allowed to change <strong>any </strong>single land cell <code>(1)</code> into a water cell <code>(0)</code>.</p>

<p>Return <em>the minimum number of days to disconnect the grid</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/12/24/land1.jpg" style="width: 500px; height: 169px;" />
<pre>
<strong>Input:</strong> grid = [[0,1,1,0],[0,1,1,0],[0,0,0,0]]

<strong>Output:</strong> 2
<strong>Explanation:</strong> We need at least 2 days to get a disconnected grid.
Change land grid[1][1] and grid[0][2] to water and get 2 disconnected island.
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/12/24/land2.jpg" style="width: 404px; height: 85px;" />
<pre>
<strong>Input:</strong> grid = [[1,1]]
<strong>Output:</strong> 2
<strong>Explanation:</strong> Grid of full water is also disconnected ([[1,1]] -&gt; [[0,0]]), 0 islands.
</pre>

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

<ul>
	<li><code>m == grid.length</code></li>
	<li><code>n == grid[i].length</code></li>
	<li><code>1 &lt;= m, n &lt;= 30</code></li>
	<li><code>grid[i][j]</code> is either <code>0</code> or <code>1</code>.</li>
</ul>

class Solution {
public:
    int dx[4] = {0, 1, 0, -1};
    int dy[4] = {-1, 0, 1, 0};
    void dfs(vector<vector<int>>& a, int n, int m, int x, int y) {
        stack<pair<int, int>> st;
        st.emplace(x, y);
        a[x][y] = 0;
        while (!st.empty()) {
            auto [u, v] = st.top();
            st.pop();
            for (int i = 0; i < 4; ++i) {
                x = u + dx[i];
                y = v + dy[i];
                if (x < 0 || x >= n || y < 0 || y >= m || !a[x][y]) {
                    continue;
                }
                st.emplace(x, y);
                a[x][y] = 0;
            }
        }
    }

    int countIslands(vector<vector<int>> a, int n, int m) {
        int res = 0;
        for (int i = 0; i < n; ++i) {
            for (int j = 0; j < m; ++j) {
                if (a[i][j]) {
                    ++res;
                    dfs(a, n, m, i, j);
                }
            }
        }
        return res;
    }

    int minDays(vector<vector<int>>& grid) {
        int n = grid.size();
        int m = grid[0].size();
        if (countIslands(grid, n, m) != 1)
            return 0;
        for (int i = 0; i < n; ++i) {
            for (int j = 0; j < m; ++j) {
                if (grid[i][j] == 1) {
                    grid[i][j] = 0;
                    if (countIslands(grid, n, m) != 1)
                        return 1;
                    grid[i][j] = 1;
                }
            }
        }
        return 2;
    }
};
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