<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>

struct Hash {
   size_t operator() (const std::pair<size_t, size_t> &p) const {
     return std::hash<size_t>()(p.first) ^ std::hash<size_t>()(p.second);
   }
};

class Solution {
    std::unordered_set<std::pair<size_t, size_t>, Hash> visited;

public:
    int numIslands(vector<vector<char>>& grid) {
        int result = 0;
        for (size_t i = 0; i < grid.size(); ++i)
        {
            for (size_t j = 0; j < grid[0].size(); ++j)
            {
                if (grid[i][j] == '0')
                    continue;
                if (visited.contains({i, j}))
                    continue;
                explore(i, j, grid);
                ++result;
            }
        }
        return result;
    }

    void explore(size_t i, size_t j, const vector<vector<char>>& grid)
    {
        if (i >= grid.size() || j >= grid[0].size())
            return;
        if (grid[i][j] == '0')
            return;
        if (visited.contains({i, j}))
            return;
        visited.insert({i, j});

        // explore(i - 1, j - 1, grid);
        explore(i, j - 1, grid);
        // explore(i + 1, j - 1, grid);

        explore(i - 1, j, grid);
        explore(i + 1, j, grid);

        // explore(i - 1, j + 1, grid);
        explore(i, j + 1, grid);
        // explore(i + 1, j + 1, grid);
    }
};
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