<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 n;
    vector<pair<int, int>> island;
    bool **collected;
    void expand(int i, int j, int k, vector<vector<int>>& grid){
        if(grid[i][j] == 0 || collected[i][j]) return;
        island.push_back({i, j});
        collected[i][j] = 1;
        if(i - 1 >= 0) expand(i - 1, j, k, grid);
        if(i + 1 < n) expand(i + 1, j, k, grid);
        if(j - 1 >= 0) expand(i, j - 1, k, grid);
        if(j + 1 < n) expand(i, j + 1, k, grid);
    }

    int shortestBridge(vector<vector<int>>& grid) {
        vector<pair<int, int>> island1;
        vector<pair<int, int>> island2;
        n = grid.size();
        collected = new bool*[n];
        for(int i = 0; i < n; i++) collected[i] = new bool[n];
        for(int i = 0; i < n; i++){
            for(int j = 0; j < n; j++) collected[i][j] = false;
        }
        bool found = false;
        for(int i = 0; i < n; i++){
            for(int  j = 0; j < n; j++){
                if(grid[i][j] == 1 && !collected[i][j]){
                    expand(i, j, 0, grid);
                    island1 = island;
                    island.clear();
                    found = true;
                    break;
                }
            }
            if(found) break;
        }
        //found = false;
        for(int i = 0; i < n; i++){
            for(int  j = 0; j < n; j++){
                if(grid[i][j] == 1 && !collected[i][j]){
                    island2.push_back({i, j});
                    // expand(i, j, 1, grid);
                    // island2 = island;
                    // found = true;
                    // break;
                }
            }
            //if(found) break;
        }
        int min_dist = 2 * n - 1;
        int dist;
        for(pair<int, int> p1:island1){
            for(pair<int, int> p2:island2){
                dist = abs(p1.first - p2.first) + abs(p1.second - p2.second);
                if(dist < min_dist) min_dist = dist;
            }
        }
        return min_dist - 1;
    }
};
27085