<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:
    unordered_set<string> visited;
    int ROWS;
    int COLS;
    int numIslands(vector<vector<char>>& grid) {
        ROWS = grid.size();
        COLS = grid[0].size();
        
        int count = 0;
        for (int i=0; i<ROWS; ++i) {
            for (int j=0; j<COLS; ++j) {
                string str = to_string(i) + "," + to_string(j);
                if (grid[i][j] == '1' && visited.find(str) == visited.end()) {
                    dfs(i, j, grid);
                    count++;
                }
            }
        }
        for (const auto &x : visited)
            cout << x << endl;
        return count;
    }

    void dfs(const int x, const int y, vector<vector<char>>& grid) {
        string str = to_string(x) + "," + to_string(y);
        if (x>=ROWS || 
            y>=COLS || 
            x<0 || y<0 || visited.find(str) != visited.end() || grid[x][y]=='0') {
            return;
        }
        visited.insert(str);
        dfs(x+1, y, grid);
        dfs(x-1, y, grid);
        dfs(x, y+1, grid);
        dfs(x, y-1, grid);
    }
};
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