<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:
    def numIslands(self, grid: List[List[str]]) -> int:
        isl_map = [[0 for _ in range(len(grid[0]))] for _ in range(len(grid))]
        isl_num = 1

        def explore(x, y, grid, isl, val):
            possible = [(x, y)]
            visited = {()}
            while possible:
                x, y = possible.pop()
                if (x > -1
                    and y > -1
                    and x < len(grid)
                    and y < len(grid[0])
                        and grid[x][y] == '1'):
                    isl[x][y] = val
                    visited.add((x, y))
                    if (x+1, y) not in visited:
                        possible.append((x+1, y))
                    if (x, y+1) not in visited:
                        possible.append((x, y+1))
                    if (x-1, y) not in visited:
                        possible.append((x-1, y))
                    if (x, y-1) not in visited:
                        possible.append((x, y-1))

        # assign numbers
        for i, row in enumerate(grid):
            for j, col in enumerate(row):
                if col == '1' and isl_map[i][j] == 0:
                    explore(i, j, grid, isl_map, isl_num)
                    isl_num += 1

        return isl_num - 1
19088