<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 {

    int res_{0};
public:

    vector<pair<int, int>> GetVaildNeighbors(vector<vector<char>>& grid, int x, int y) {
        int m = grid.size();
        int n = grid[0].size();
        vector<pair<int,int>> neighbors;
        if(x > 0 and grid[x-1][y] == '1') {
            neighbors.emplace_back(x-1, y);
        }
        if(y > 0 and grid[x][y-1] == '1') {
            neighbors.emplace_back(x, y-1);
        }
        if(x < m-1 and grid[x+1][y] == '1') {
            neighbors.emplace_back(x+1, y);
        } 
        if(y < n-1 and grid[x][y+1] == '1') {
            neighbors.emplace_back(x, y+1);
        }
        return neighbors;
    }

    void DFS(vector<vector<char>>& grid, int x, int y) {
        grid[x][y] = '0';
        auto neighbor = GetVaildNeighbors(grid, x, y);
        for(auto [n_x, n_y] : neighbor) {
            DFS(grid, n_x, n_y);
        }
    }

    int numIslands(vector<vector<char>>& grid) {
        for(int i = 0; i < grid.size(); i++) {
            for(int j = 0; j < grid[0].size(); j++) {
                if(grid[i][j] == '1') {
                    res_++;
                    DFS(grid, i ,j);
                }
            }
        }
        return res_;
    }
};
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