<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) {
        int islands = 0;
        int rows = grid.size();
        int cols = grid[0].size();
        unordered_set<string> visited;

        vector<pair<int, int>> directions = {{1, 0}, {-1, 0}, {0, 1}, {0, -1}};

        for (int r = 0; r < rows; r++) {
            for (int c = 0; c < cols; c++) {
                if (grid[r][c] == '1' && visited.find(to_string(r) + "," + to_string(c)) == visited.end()) {
                    islands++;
                    bfs(grid, r, c, visited, directions, rows, cols);
                }
            }
        }

        return islands;        
    }

private:
    void bfs(vector<vector<char>>& grid, int r, int c, unordered_set<string>& visited, vector<pair<int, int>>& directions, int rows, int cols) {
        queue<pair<int, int>> q;
        visited.insert(to_string(r) + "," + to_string(c));
        q.push({r, c});

        while (!q.empty()) {
            auto [row, col] = q.front();
            q.pop();

            for (auto [dr, dc] : directions) {
                int nr = row + dr;
                int nc = col + dc;
                if (nr >= 0 && nr < rows && nc >= 0 && nc < cols && grid[nr][nc] == '1' && visited.find(to_string(nr) + "," + to_string(nc)) == visited.end()) {
                    q.push({nr, nc});
                    visited.insert(to_string(nr) + "," + to_string(nc));
                }
            }
        }
    }
};    
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