42. Trapping Rain Water

思路转自https://blog.youkuaiyun.com/makuiyu/article/details/43650365：

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

        }
        return ans;
        }
};