<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 {
    stack<std::pair<int, int>> find, fill;
    set<std::pair<int, int>> check_set = {{-1, 0}, {1, 0}, {0, -1}, {0, 1}};
public:
    void fillIsland(vector<vector<char>> &grid){
        while(fill.size() > 0){
            auto [ci, cj] = fill.top();
            fill.pop();

            for(auto &[x, y] : check_set){
                int i = ci + x, j = cj + y;
                if(i >= 0 && i < grid.size() && j >= 0 && j < grid[0].size()){
                    if(grid[i][j] == '1'){
                        grid[i][j] = '#';
                        fill.push({i, j});
                    } else if (grid[i][j] == '0'){
                        grid[i][j] = '#';
                        find.push({i, j});
                    }
                }
            }
        }
    }

    int numIslands(vector<vector<char>>& grid) {
        int cnt_islands = 0;

        if(grid[0][0] == '1'){
            fill.push({0, 0});
            ++cnt_islands;
            fillIsland(grid);
        } else {
            find.push({0, 0});
        }
        grid[0][0] = '#';

        while(find.size() > 0){
            auto [ci, cj] = find.top();
            find.pop();

            for(auto &[x, y] : check_set){
                int i = ci + x, j = cj + y;
                if(i >= 0 && i < grid.size() && j >= 0 && j < grid[0].size()){
                    if(grid[i][j] == '1'){
                        ++cnt_islands;
                        grid[i][j] = '#';
                        fill.push({i, j});
                        fillIsland(grid);
                    } else if (grid[i][j] == '0'){
                        grid[i][j] = '#';
                        find.push({i, j});
                    }
                }
            }
        }

        return cnt_islands;
    }
};
17578