<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:
    struct Compare {
        bool operator()(vector<int> const & a, vector<int> const & b)
        { return a[0] > b[0]; }
    };
    int trapRainWater(vector<vector<int>>& heightMap) {
        int m=heightMap.size(), n= heightMap[0].size();
        int water=0;
        priority_queue<vector<int>, vector<vector<int>>, Compare > pq;
        for(int i=0;i<m;i++){
            for(int j=0;j<n;j++){
                if(i==0 || j==0 || j==n-1 || i==m-1){
                    pq.push({heightMap[i][j],i,j});
                    heightMap[i][j]=-1;
                }
            }
        }
        int level=0;
        while(!pq.empty()){
            vector<int> curr=pq.top();
            //cout<<curr[0]<<endl;
            level=max(curr[0],level);
            cout<<level<<endl;
            cout<<curr[0]<<endl;
            pq.pop();
            cout<<endl;
            int i=curr[1];
            int j=curr[2];
            if(i-1>=0 && heightMap[i-1][j]!=-1 ){
                if(heightMap[i-1][j]<level) water+=level-heightMap[i-1][j];
                pq.push({heightMap[i-1][j],i-1,j});
                heightMap[i-1][j]=-1;
            }
            cout<<water<<endl;
            if(i+1<m && heightMap[i+1][j]!=-1){
                if(heightMap[i+1][j]<level) water+=level-heightMap[i+1][j];
                pq.push({heightMap[i+1][j],i+1,j});
                heightMap[i+1][j]=-1;
            }
            cout<<water<<endl;
            if(j-1>=0 && heightMap[i][j-1]!=-1){
                if(heightMap[i][j-1]<level) water+=level-heightMap[i][j-1];
                pq.push({heightMap[i][j-1],i,j-1});
                heightMap[i][j-1]=-1;
            }
            cout<<water<<endl;
            if(j+1<n && heightMap[i][j+1]!=-1){
                if(heightMap[i][j+1]<level) water+=level-heightMap[i][j+1];
                pq.push({heightMap[i][j+1],i,j+1});
                heightMap[i][j+1]=-1;
            }
            cout<<water<<endl;
        }
        return water;
    }
};
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