<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) {
        const size_t height = grid.size();
        const size_t width = grid.back().size();
        
        int count = 0;
        vector<vector<bool>> visited(height, vector<bool>(width));
        for (size_t row = 0; row < height; ++row) {
            for (size_t col = 0; col < width; ++col) {
                if (visited[row][col] || grid[row][col] == '0') {
                    continue;
                }
                
                ++count;
                deque<pair<size_t, size_t>> queue{{row, col}};
                while (!queue.empty()) {
                    auto [land_row, land_col] = queue.front();
                    queue.pop_front();
                    visited[land_row][land_col] = true;
                    for (auto [dy, dx] : vector<pair<int, int>>{{1, 0}, {-1, 0}, {0, 1}, {0, -1}}) {
                        size_t next_row = land_row + dy;
                        size_t next_col = land_col + dx;
                        if (next_row >= height || next_col >= width) {
                            continue;
                        }
                        if (visited[next_row][next_col]) {
                            continue;
                        }
                        if (grid[next_row][next_col] == '0') {
                            continue;
                        }
                        queue.emplace_back(next_row, next_col);
                        visited[next_row][next_col] = true;
                    }
                }
            }
        }
        
        return count;
    }
};
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