[LintCode] Trapping rain water II

Given n x m non-negative integers representing an elevation map 2d where the area of each cell is 1 x 1, compute how much water it is able to trap after raining.

Example

Given 5*4 matrix

[12,13,0,12]
[13,4,13,12]
[13,8,10,12]
[12,13,12,12]
[13,13,13,13]

return 14.

struct MyPoint{
  int x, y, h;
  MyPoint(int xx, int yy, int hh):x(xx), y(yy), h(hh){}
};

struct Cmp{
  bool operator()(const MyPoint &p1, const MyPoint &p2){
      return p1.h > p2.h;
  }
};

class Solution {
public:
    /**
     * @param heights: a matrix of integers
     * @return: an integer
     */
    int trapRainWater(vector<vector<int> > &heights) {
        int m = heights.size();
        if(m < ) return ;
        int n = heights[].size();
        if(n < ) return ;

        int waterNum = ;
        vector<vector<bool> > visit(m, vector<bool>(n, false));
        priority_queue<MyPoint, vector<MyPoint>, Cmp> MyPointQue;

        for(int i = ;i < n;++i){
            MyPointQue.push(MyPoint(, i, heights[][i]));
            MyPointQue.push(MyPoint(m - , i, heights[m - ][i]));
            visit[][i] = true;
            visit[m - ][i] = true;
        }

        for(int j = ;j < m - ;++j){
            MyPointQue.push(MyPoint(j, , heights[j][]));
            MyPointQue.push(MyPoint(j, n - , heights[j][n - ]));
            visit[j][] = true;
            visit[j][n - ] = true;
        }

        const int detX[] = {, , , -}, detY[] = {, -, , };
        while(MyPointQue.size() > ){
            MyPoint p = MyPointQue.top();
            MyPointQue.pop();

            for(int i = ;i < ;++i){
                int xx = p.x + detX[i], yy = p.y + detY[i];
                if(xx <  || xx >= m || yy <  || yy >= n) continue;
                if(visit[xx][yy]) continue;
                else{
                    int h = heights[xx][yy];
                    if(heights[xx][yy] < p.h){
                        h = p.h;
                        waterNum += p.h - heights[xx][yy];
                    }
                    MyPointQue.push(MyPoint(xx, yy, h));
                    visit[xx][yy] = true;
                }
            }
        }
        return waterNum;
    }
};