<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 n,m;
    bool vis[305][305];
    vector<pair<int,int>> d = {{0,1}, {0,-1}, {-1,0}, {1,0}};
    bool valid(int ci, int cj)
    {
        if(ci >= 0 && ci <n && cj >= 0 && cj < m) return true;
        else return false;
    }
    void dfs(int si, int sj, vector<vector<char>>& grid)
    {
        vis[si][sj] = true;
        for(int i = 0; i < 4; i++)
        {
            int ci = si + d[i].first;
            int cj = sj + d[i].second;
            if(valid(ci,cj) && !vis[ci][cj] && grid[ci][cj] == '1')
            {
                dfs(ci,cj, grid);
            }
        }
    }
    int numIslands(vector<vector<char>>& grid) {
        n = grid.size();
        m = grid[0].size();
        memset(vis, false, sizeof(vis));
        int ans = 0;
        for(int i = 0; i < n; i++)
        {
            for(int j = 0; j < m; j++)
            {
                if(!vis[i][j] && grid[i][j] == '1')
                {
                    dfs(i,j, grid);
                    ans++;
                }
            }
        }
        return ans;
    }
};
9901