<p>You are given an <code>n x n</code> binary matrix <code>grid</code> where <code>1</code> represents land and <code>0</code> represents water.</p>

<p>An <strong>island</strong> is a 4-directionally connected group of <code>1</code>&#39;s not connected to any other <code>1</code>&#39;s. There are <strong>exactly two islands</strong> in <code>grid</code>.</p>

<p>You may change <code>0</code>&#39;s to <code>1</code>&#39;s to connect the two islands to form <strong>one island</strong>.</p>

<p>Return <em>the smallest number of </em><code>0</code><em>&#39;s you must flip to connect the two islands</em>.</p>

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

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

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

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

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

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

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

<ul>
	<li><code>n == grid.length == grid[i].length</code></li>
	<li><code>2 &lt;= n &lt;= 100</code></li>
	<li><code>grid[i][j]</code> is either <code>0</code> or <code>1</code>.</li>
	<li>There are exactly two islands in <code>grid</code>.</li>
</ul>

class Solution {
public:
    int shortestBridge(vector<vector<int>>& grid) {

        int m = grid.size(), n = grid[0].size();

        queue<vector<int>> q;
        bool flag = false;
        for(int i=0;i<m;i++) {
            for(int j=0;j<n;j++) {
                if(grid[i][j]==1) {
                    dfs(grid,i,j,m,n,q);
                    flag = true;
                    break;
                }
            }
            if(flag) break;
        }

        while(!q.empty()) {
            int row = q.front()[0];
            int col = q.front()[1];
            int dist = q.front()[2];

            q.pop();

            if(row<0 || col<0 || row>=m || col>=n || grid[row][col]==-2)
            continue;

            if(grid[row][col]==1) return dist-1;

            grid[row][col]=-2;

            q.push({row+1,col,dist+1});
            q.push({row-1,col,dist+1});
            q.push({row,col+1,dist+1});
            q.push({row,col-1,dist+1});
        }  

        return -1;
    }

    void dfs(vector<vector<int>>& grid, int row, int col, int m,int n,queue<vector<int>> &q) {
        if(row<0 || col<0 || row>=m || col>=n || grid[row][col]==0 || grid[row][col]==-1) return;

        grid[row][col] = -1;
        q.push({row,col,0});

        dfs(grid,row+1,col,m,n,q);
        dfs(grid,row-1,col,m,n,q);
        dfs(grid,row,col+1,m,n,q);
        dfs(grid,row,col-1,m,n,q);
    }
};
51055