<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:
    void get_island(vector<vector<int>>& a, int i, int j, int n, queue<vector<int>>& q)
{
    if (i < 0 || i == n || j < 0 || j == n || 1 != a[i][j])
    {
        return;
    }

    a[i][j] = 2;
    q.push({ i, j });

    get_island(a, i + 1, j, n,  q);
    get_island(a, i - 1, j, n,  q);
    get_island(a, i, j+1, n,  q);
    get_island(a, i, j-1, n,  q);
}

bool is_pos_v(int i, int j, int n)
{
    return i >= 0 && i < n && j >= 0 && j < n;
}


int shortestBridge(vector<vector<int>>& a) {
    const int n = a.size();
    queue<vector<int>>q;

    for (int i = 0; i < n; ++i)
    {
        for (int j = 0; j < n; ++j)
        {
            if (1 == a[i][j])
            {
                get_island(a, i, j, n, q);
                break;
            }
        }
        if (q.size() > 0) break;
    }

    vector<vector<int>>change{{1, 0}, {-1, 0}, {0, 1}, {0, -1}};
    int ret = 0;
    while (!q.empty())
    {
        int size = q.size();
        for (int i = 0; i < size; ++i)
        {
            vector<int>pos = q.front();
            q.pop();

            for (int j = 0; j < 4; ++j)
            {
                int x = pos[0] + change[j][0];
                int y = pos[1] + change[j][1];
                if (is_pos_v(x, y, n) && 2 != a[x][y])
                {
                    if (1 == a[x][y])
                    {
                        return ret;
                    }
                    q.push({ x, y });
                    a[x][y] = 2;
                }
            }
        }

        ++ret;
    }

    return ret;
}

};
35755