<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>

#include <vector>
#include <deque>
#include <string>
class Solution {
public:
    int numIslands(vector<vector<char>>& grid) {
        int count = 0;
        int rows = grid.size();
        if (rows == 0) return 0;
        int cols = grid[0].size();
        
        // Build the array of explored and unexplored nodes
        vector<vector<bool>> E(rows, vector<bool>(cols, false));
        
        for (int i = 0; i < rows; ++i) {
            for (int j = 0; j < cols; ++j) {
                if (grid[i][j] == '1' && !E[i][j]) {
                    count++;
                    
                    // Perform BFS on the node
                    deque<pair<int, int>> q;
                    q.push_back({i, j});
                    
                    while (!q.empty()) {
                        pair<int, int> curr = q.back();
                        q.pop_back();
                        E[curr.first][curr.second] = true;
                        
                        // Explore neighboring cells
                        if (curr.first - 1 >= 0 && grid[curr.first - 1][curr.second] == '1' && !E[curr.first - 1][curr.second]) {
                            q.push_back({curr.first - 1, curr.second});
                        }
                        if (curr.second - 1 >= 0 && grid[curr.first][curr.second - 1] == '1' && !E[curr.first][curr.second - 1]) {
                            q.push_back({curr.first, curr.second - 1});
                        }
                        if (curr.first + 1 < rows && grid[curr.first + 1][curr.second] == '1' && !E[curr.first + 1][curr.second]) {
                            q.push_back({curr.first + 1, curr.second});
                        }
                        if (curr.second + 1 < cols && grid[curr.first][curr.second + 1] == '1' && !E[curr.first][curr.second + 1]) {
                            q.push_back({curr.first, curr.second + 1});
                        }
                    }
                }
            }
        }
        
        return count;
    }
};
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