<p>You are given an <code>n x n</code> binary matrix <code>grid</code>. You are allowed to change <strong>at most one</strong> <code>0</code> to be <code>1</code>.</p>

<p>Return <em>the size of the largest <strong>island</strong> in</em> <code>grid</code> <em>after applying this operation</em>.</p>

<p>An <strong>island</strong> is a 4-directionally connected group of <code>1</code>s.</p>

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

<pre>
<strong>Input:</strong> grid = [[1,0],[0,1]]
<strong>Output:</strong> 3
<strong>Explanation:</strong> Change one 0 to 1 and connect two 1s, then we get an island with area = 3.
</pre>

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

<pre>
<strong>Input:</strong> grid = [[1,1],[1,0]]
<strong>Output:</strong> 4
<strong>Explanation: </strong>Change the 0 to 1 and make the island bigger, only one island with area = 4.</pre>

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

<pre>
<strong>Input:</strong> grid = [[1,1],[1,1]]
<strong>Output:</strong> 4
<strong>Explanation:</strong> Can&#39;t change any 0 to 1, only one island with area = 4.
</pre>

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

<ul>
	<li><code>n == grid.length</code></li>
	<li><code>n == grid[i].length</code></li>
	<li><code>1 &lt;= n &lt;= 500</code></li>
	<li><code>grid[i][j]</code> is either <code>0</code> or <code>1</code>.</li>
</ul>

class Solution {
public:
    vector<vector<int>> vis ;
    int dfs(int i, int j, vector<vector<int>> &grid){
        int n = grid.size() ;
        
        if(i < 0 || j < 0 || i >= n || j >= n || vis[i][j] == 1 || grid[i][j] != 1){
            return 0 ;
        }
        
        vis[i][j] = 1 ;
        
        int a1 = dfs(i + 1, j, grid) ;
        int a3 = dfs(i, j + 1, grid) ;
        int a2 = dfs(i - 1, j, grid) ;
        int a4 = dfs(i, j - 1, grid) ;
        // vis[i][j] = 0 ;
        return 1 + a1 + a2 + a3 + a4 ;
    }
    void dfsReset(int i, int j, vector<vector<int>> &grid){
        int n = grid.size() ;
        
        if(i < 0 || j < 0 || i >= n || j >= n || vis[i][j] == 0 || grid[i][j] != 1){
            return ;
        }
        // cout<<"Hi : "<<i<<" "<<j<<endl ;
        vis[i][j] = 0 ;
        dfsReset(i + 1, j, grid) ;
        dfsReset(i, j + 1, grid) ;
        dfsReset(i - 1, j, grid) ;
        dfsReset(i, j - 1, grid) ;
        // vis[i][j] = 0 ;
        return ;
    }
    int largestIsland(vector<vector<int>>& grid) {
        int n = size(grid) ;
        vis.resize(n , vector<int>(n, 0)) ;
        int area = INT_MIN ;
        for(int i = 0 ; i < n ; i++){
            for(int j = 0 ; j < n ; j++){
                if(grid[i][j] == 0){
                    
                    grid[i][j] =  1 ;
                    int ar = dfs(i, j, grid) ;
                    
                    dfsReset(i, j, grid) ;
                    
                    // for(auto &a : vis){
                    //     for(auto &b : a) cout<<b<<" ";
                    //     cout<<endl ;
                    // }
                    // cout<<ar<<endl ;
                    // vis.clear() ;
                    // vis.resize(n, vector<int>(n, 0)) ;
                    
                    area = max(area, ar) ;
                    grid[i][j] = 0 ;
                }
            }
        }
        return (area == INT_MIN) ? dfs(0, 0, grid) : area ;
    }
};
64659