<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 count = 0;

  int m = grid.size();
  int n = grid[0].size();

  int visited[m * n];
  for (size_t i = 0; i < m*n; i++) {
    visited[i] = 0;
  }

  std::queue<std::pair<int, int>> bfs_queue;

  for (size_t i = 0; i < m; i++) {
    for (size_t j = 0; j < n; j++) {
      int idx = j + i * n;

      if (visited[idx] == 1) {
        continue;
      }

      if (grid[i][j] != '1') {
        continue;
      }

      bfs_queue.push(std::make_pair(i, j));
      count++;
      
      while (!bfs_queue.empty()) {
        auto idx_pair = bfs_queue.front();
        bfs_queue.pop();

        if (idx_pair.first < 0 || idx_pair.first >= m || idx_pair.second < 0 ||idx_pair.second >= n) {
          continue;
        }

        if (grid[idx_pair.first][idx_pair.second] != '1') {
          continue;
        }

        int idx = idx_pair.second + idx_pair.first * n;
        if (visited[idx] == 1) {
          continue;
        }

        bfs_queue.push(std::make_pair(idx_pair.first, idx_pair.second - 1));
        bfs_queue.push(std::make_pair(idx_pair.first + 1, idx_pair.second));
          bfs_queue.push(std::make_pair(idx_pair.first - 1, idx_pair.second));
        bfs_queue.push(std::make_pair(idx_pair.first, idx_pair.second + 1));
        visited[idx] = 1;
      }
    }
  }
  return count;
    }
};
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