407 Trapping Rain Water II

Given an m x n matrix of positive integers representing the height of each unit cell in a 2D elevation map, compute the volume of water it is able to trap after raining.

Example:

Given the following 3x6 height map:
[
  [1,4,3,1,3,2],
  [3,2,1,3,2,4],
  [2,3,3,2,3,1]
]

Return 4.

The above image represents the elevation map [[1,4,3,1,3,2],[3,2,1,3,2,4],[2,3,3,2,3,1]] before the rain.

After the rain, water is trapped between the blocks. The total volume of water trapped is 4.

Constraints:

• 1 <= m, n <= 110

• 0 <= heightMap[i][j] <= 20000

这一题的思路跟42 Trapping Rain Water相似，主要是找围着某一点的柱子里，最矮的那一个。然后能装多少水，就是那个跟被围着的点的差。我们还得边检查，边更新现在围住的“边”的最低高度。这一题因为要在一串数字里（围着的边）不断地找最小值，所以用了堆