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.
思路：
代码：
class Solution {
public:
int trap(vector<int>& height) {
int length = height.size();
int secHeight = 0, left = 0, right = length -1, ans = 0;
while(left < right) {
if (height[left] < height[right])
{
secHeight = max(secHeight, height[left]);
ans += secHeight - height[left];
left ++;
}
else
{
secHeight = max(secHeight, height[right]);
ans += secHeight - height[right];
right --;
}
}
return ans;
}
};