<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:
    set<pair<int,int>> st;//land cells list
    int n,m;
    bool isValid(int i, int j){
        if(i>=0 && j>=0 && i<n && j<m) return true;
        return false;
    }
    void dfs(int r, int c, vector<vector<int>> &grid, vector<vector<int>> &vis){
        vis[r][c]=1;
        if(st.find({r,c})==st.end()) st.insert({r,c});
        int del[4] = {-1,0,1,0};
        for(int i=0; i<4;i++){
            int nr = r + del[i];
            int nc = c + del[(i+1)%4];
            if(isValid(nr,nc) && grid[nr][nc]==1 && vis[nr][nc]==0){
                dfs(nr, nc, grid, vis);
            }
        }
    }
    int minDays(vector<vector<int>>& grid) {
        n = grid.size();
        m = grid[0].size();
        vector<vector<int>> vis(n, vector<int> (m,0));
        
        int component = 0;
        for(int i=0; i<n; i++){
            for(int j=0; j<m; j++){
                if(grid[i][j]==1 && vis[i][j]==0){
                    dfs(i, j, grid, vis);
                    component++;
                }
            }
        }
        if(component==0||component>1)return 0;
        // for single component
        for(auto it: st){
            int x = it.first, y=it.second;
            grid[x][y] = 0;

            vector<vector<int>> vis2(n, vector<int> (m,0));
        
            int component = 0;
            for(int i=0; i<n; i++){
                for(int j=0; j<m; j++){
                    if(grid[i][j]==1 && vis2[i][j]==0){
                        dfs(i, j, grid, vis2);
                        component++;
                    }
                }
            }
            if(component!=1) return 1;

            grid[x][y]=1;
        }
        return 2;
    }
};
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