<p>Given an <code>m x n</code> integer matrix <code>heightMap</code> representing the height of each unit cell in a 2D elevation map, return <em>the volume of water it can trap after raining</em>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/04/08/trap1-3d.jpg" style="width: 361px; height: 321px;" />
<pre>
<strong>Input:</strong> heightMap = [[1,4,3,1,3,2],[3,2,1,3,2,4],[2,3,3,2,3,1]]
<strong>Output:</strong> 4
<strong>Explanation:</strong> After the rain, water is trapped between the blocks.
We have two small ponds 1 and 3 units trapped.
The total volume of water trapped is 4.
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/04/08/trap2-3d.jpg" style="width: 401px; height: 321px;" />
<pre>
<strong>Input:</strong> heightMap = [[3,3,3,3,3],[3,2,2,2,3],[3,2,1,2,3],[3,2,2,2,3],[3,3,3,3,3]]
<strong>Output:</strong> 10
</pre>

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

<ul>
	<li><code>m == heightMap.length</code></li>
	<li><code>n == heightMap[i].length</code></li>
	<li><code>1 &lt;= m, n &lt;= 200</code></li>
	<li><code>0 &lt;= heightMap[i][j] &lt;= 2 * 10<sup>4</sup></code></li>
</ul>

class Solution {
public:
    class compare{
        public:
        bool operator()(pair<int,pair<int,int>> p1, pair<int,pair<int,int>> p2){
            return p1.first > p2.first;
        }
    };
    int trapRainWater(vector<vector<int>>& h) {
        int n = h.size(), m = h[0].size();
        vector<vector<int>> vis(n,vector<int>(m,0));
        priority_queue<pair<int,pair<int,int>>, vector<pair<int,pair<int,int>>>, compare> pq;
        
        for(int i = 0;i<n;i++){
            for(int j = 0;j<m;j++){
                if(i == 0 || j== 0 || j == m-1 || i == n-1){
                    pq.push({h[i][j],{i,j}});
                    vis[i][j] = 1;
                }
            }
        }
        vector<int> dx = {-1,0,1,0};
        vector<int> dy = {0,1,0,-1};
        int ans = 0;

        auto isValid = [=](int x, int y){
            if(x<0 || y<0 || x>=n || y>=m) return false;
            return true; 
        };
        while(!pq.empty()){
            auto x = pq.top();
            pq.pop();
            int height = x.first;
            int i = x.second.first;
            int j = x.second.second;
            ans += height-h[i][j];
            for(int k = 0;k<4;k++){
                int x = i+dx[k];
                int y = j+dy[k];
                if(isValid(x,y) && !vis[x][y]){
                    vis[x][y] = 1;
                    pq.push({max(height,h[x][y]),{x,y}});
                }
            }
        }
        return ans;
    }
};
18881