<p>Given an <code>m x n</code> 2D binary grid <code>grid</code> which represents a map of <code>&#39;1&#39;</code>s (land) and <code>&#39;0&#39;</code>s (water), return <em>the number of islands</em>.</p>

<p>An <strong>island</strong> is surrounded by water and is formed by connecting adjacent lands horizontally or vertically. You may assume all four edges of the grid are all surrounded by water.</p>

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

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

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

<pre>
<strong>Input:</strong> grid = [
  [&quot;1&quot;,&quot;1&quot;,&quot;0&quot;,&quot;0&quot;,&quot;0&quot;],
  [&quot;1&quot;,&quot;1&quot;,&quot;0&quot;,&quot;0&quot;,&quot;0&quot;],
  [&quot;0&quot;,&quot;0&quot;,&quot;1&quot;,&quot;0&quot;,&quot;0&quot;],
  [&quot;0&quot;,&quot;0&quot;,&quot;0&quot;,&quot;1&quot;,&quot;1&quot;]
]
<strong>Output:</strong> 3
</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;= 300</code></li>
	<li><code>grid[i][j]</code> is <code>&#39;0&#39;</code> or <code>&#39;1&#39;</code>.</li>
</ul>

class Solution {
public:
    int numIslands(vector<vector<char>>& grid) {
        queue<pair<int,int>> q;
        int m=grid.size(),n=grid[0].size();
        std::vector<std::vector<bool>> visited(m, std::vector<bool>(n, false));
        //vector<vector<bool>> visited=(m,vector<bool>(n,false));
        vector<int> dxs={0,1,0,-1};
        vector<int> dys={1,0,-1,0};
        int num=0;
        for(int i=0;i<m;i++)
        {
            for(int j=0;j<n;j++)
            {
                if(grid[i][j]=='1'&&!visited[i][j])   //该区域是陆地且还未访问过
                {
                    q.push(make_pair(i,j));
                    visited[i][j] = true;
                    while(!q.empty())
                    {
                        pair<int,int> coordinate=q.front();
                        q.pop();
                        //visited[coordinate.first][coordinate.second]=true;
                        for(int s=0;s<dxs.size();s++)
                        {
                            int x=coordinate.first+dxs[s];
                            int y=coordinate.second+dys[s];
                            if(x>=0&&x<m&&y>=0&&y<n&&grid[x][y]=='1'&&!visited[x][y])    //四周格子在范围之内
                            {
                                q.push(make_pair(x,y));
                                visited[x][y] = true;
                            }
                        }
                    }
                    num++;
                }
            }
        }
        return num;
    }
};
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