<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) {
        vector<vector<bool>> visited;
        vector<pair<int, int>> directions;
        int ans = 0;
        directions.push_back(make_pair(1, 0));
        directions.push_back(make_pair(-1, 0));
        directions.push_back(make_pair(0, -1));
        directions.push_back(make_pair(0, 1));

        visited.resize(grid.size());
        for(int i = 0; i < visited.size(); i++){
            visited[i].resize(grid[0].size());
        }

        for(int i = 0; i < grid.size(); i++){
            for(int j = 0; j < grid[i].size(); j++){
                if(!visited[i][j] && grid[i][j] == '1'){
                    BFS(i, j, grid, visited, directions);
                    ans++;
                }
            }
        }
        return ans;
    }

    void BFS(int a, int b, vector<vector<char>>& grid, vector<vector<bool>>& visited, vector<pair<int, int>>& directions){
        queue<pair<int, int>> q;
        q.push(make_pair(a, b));
        while(!q.empty()){
            a = q.front().first;
            b = q.front().second; q.pop();
            if(a >= grid.size() || b >= grid[0].size() || a < 0 || b < 0 || grid[a][b] != '1' || visited[a][b]){
                continue;
            }
            visited[a][b] = true;
            for(int i = 0; i < directions.size(); i++){
                q.push(make_pair(directions[i].first + a, directions[i].second + b));
            }
        }
    }
};
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