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

    void visitIsland(const vector<vector<char>>& grid, int rows, int cols, int row, int col, vector<vector<bool>>& visited) {
        if (row < 0 || row >= rows || col < 0 || col >= cols || visited[row][col] || grid[row][col] == '0') {
            return;
        }
        visited[row][col] = true;
        for (const vector<int>& dir: DIRS) {
            visitIsland(grid, rows, cols, row + dir[0], col + dir[1], visited);
        }
    }
public:
    int numIslands(vector<vector<char>>& grid) {
        if (grid.size() == 0 || grid[0].size() == 0) {
            return 0;
        }
        const int rows = grid.size();
        const int cols = grid[0].size();
        vector<vector<bool>> visited(rows, vector<bool>(cols, false));
        int numIslands = 0;

        for (int row = 0; row < rows; ++row) {
            //cout << "row = " << row << endl;
            for (int col = 0; col < cols; ++col) {
                //cout << "col = " << col << endl;
                if (!visited[row][col] && grid[row][col] == '1') {
                    //cout << "++numIslands" << endl;
                    ++numIslands;
                    visitIsland(grid, rows, cols, row, col, visited);
                }
            }
        }

        return numIslands;
    }
};
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