<p>An <code>n x n</code> grid is composed of <code>1 x 1</code> squares where each <code>1 x 1</code> square consists of a <code>&#39;/&#39;</code>, <code>&#39;\&#39;</code>, or blank space <code>&#39; &#39;</code>. These characters divide the square into contiguous regions.</p>

<p>Given the grid <code>grid</code> represented as a string array, return <em>the number of regions</em>.</p>

<p>Note that backslash characters are escaped, so a <code>&#39;\&#39;</code> is represented as <code>&#39;\\&#39;</code>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2018/12/15/1.png" style="width: 200px; height: 200px;" />
<pre>
<strong>Input:</strong> grid = [&quot; /&quot;,&quot;/ &quot;]
<strong>Output:</strong> 2
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2018/12/15/2.png" style="width: 200px; height: 198px;" />
<pre>
<strong>Input:</strong> grid = [&quot; /&quot;,&quot;  &quot;]
<strong>Output:</strong> 1
</pre>

<p><strong class="example">Example 3:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2018/12/15/4.png" style="width: 200px; height: 200px;" />
<pre>
<strong>Input:</strong> grid = [&quot;/\\&quot;,&quot;\\/&quot;]
<strong>Output:</strong> 5
<strong>Explanation: </strong>Recall that because \ characters are escaped, &quot;\\/&quot; refers to \/, and &quot;/\\&quot; refers to /\.
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li><code>n == grid.length == grid[i].length</code></li>
	<li><code>1 &lt;= n &lt;= 30</code></li>
	<li><code>grid[i][j]</code> is either <code>&#39;/&#39;</code>, <code>&#39;\&#39;</code>, or <code>&#39; &#39;</code>.</li>
</ul>

class Solution {
public:
    int regionsBySlashes(vector<string>& grid) {
        int n = grid.size();
        // create a 3x3 subgrid in each square
        vector<vector<int>> pgrid (3*n, vector<int> (3*n,0));
        for (int i = 0; i <n; ++i) {
            for (int j = 0; j<n; ++j) {
                if (grid[i][j] == '\\') {
                    vector<pair<int,int>> points = {{3*i,3*j},{3*i+1,3*j},{3*i+1,3*j+1},{3*i+2,3*j+1},{3*i+2,3*j+2},{3*i+3,3*j+2}};
                    for (auto [ni,nj] : points) {
                        if (ni >= 0 && ni < 3*n && nj >= 0 && nj < 3*n) {
                            pgrid[ni][nj] = 1;
                        }
                    }
                }
                if (grid[i][j] == '/') {
                    vector<pair<int,int>> points = {{3*i,3*j+2},{3*i+1,3*j+2},{3*i+1,3*j+1},{3*i+2,3*j+1},{3*i+2,3*j},{3*i+3,3*j}};
                    for (auto [ni,nj] : points) {
                        if (ni >= 0 && ni < 3*n && nj >= 0 && nj < 3*n) {
                            pgrid[ni][nj] = 1;
                        }
                    }
                }
            }
        }
        int regions = 0;
        vector<int> dx = {-1,-1,-1,0,0,1,1,1};
        vector<int> dy = {-1,0,1,-1,1,-1,0,1};
        for (int i = 0; i<3*n; i++) {
            for (int j = 0; j<3*n; j++) {
                if (pgrid[i][j] > 0) continue; // already visited or blocked
                queue<pair<int,int>> q;
                q.push({i,j});
                pgrid[i][j] = regions+2;
                while (!q.empty()) {
                    auto [r,c] = q.front(); q.pop();
                    for (int d = 0; d < 8; ++d) {
                        int ni = r+dx[d];
                        int nj = c+dy[d];
                        if (ni >= 0 && ni < 3*n && nj >= 0 && nj < 3*n && pgrid[ni][nj] == 0) {
                            pgrid[ni][nj] = regions+2;
                            q.push({ni,nj});            
                        }
                    }
                }
                regions++;
            }
        }
        //cout << pgrid;
        return regions;
    }
};
20021