<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
{
    const std::array<int, 5> OFFSETS = {-1, 0, 1, 0, -1};

public:
    int numIslands(std::vector<std::vector<char>> &grid)
    {
        int M = grid.size(), N = grid[0].size();
        int count = 0;

        for (int r = 0; r < M; r++)
        {
            for (int c = 0; c < N; c++)
            {
                if (grid[r][c] == '0')
                    continue;

                std::stack<std::pair<int, int>> stack;
                stack.push({r, c});

                while (!stack.empty())
                {
                    auto [i, j] = stack.top();
                    grid[i][j] = '0'; // mark as visited
                    stack.pop();

                    for (int k = 0; k < OFFSETS.size() - 1; k++)
                    {
                        int ni = i + OFFSETS[k], nj = j + OFFSETS[k + 1];
                        if (ni >= 0 && ni < M && nj >= 0 && nj < N && grid[ni][nj] == '1')
                            stack.push({ni, nj});
                    }
                }

                count++;
            }
        }

        return count;
    }
};

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