LeetCode Trapping Rain Water

题目：

Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining.

For example, 
Given [0,1,0,2,1,0,1,3,2,1,2,1], return 6.

The above elevation map is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped. Thanks Marcos for contributing this image!

class Solution {
public:
	int trap(int A[], int n) {
		if (n <= 2)
			return 0;
		int maxH = 0;
		//left[i]表示i左边最高的值，right[i]表示i右边最高的值
		int *left= new int[n], *right = new int[n];
		for (int i = 0; i < n; i++) {
			maxH = max(maxH, A[i]);
			left[i] = maxH;
		}
		maxH = 0;
		for (int i = n - 1; i >= 0; i--) {
			maxH = max(maxH, A[i]);
			right[i] = maxH;
		}
		int ans = 0;
		for (int i = 0; i < n; i++)
			ans += (min(left[i], right[i]) - A[i]);
		delete[] left;
		delete[] right;
		return ans;
	}
};