方法一：按层遍历

class Solution {
public:
    int trap(vector<int>& height) {
        int Sum = accumulate(height.begin(),height.end(),0);
        int volume = 0;
        int layer = 1;
        int n = height.size();
        int left = 0 ;
        int right = n-1;        
        while(left <= right){
            while(left <= right && height[left] < layer) 
                left++;
            while(left <= right && height[right] < layer) 
                right--;
            volume += right-left+1;
            layer++;
        }
        return volume - Sum;
    }
};

方法二：正序遍历一次记录左边最大值，逆序遍历一次记录每个元素的右边最大值，每个位置的存水量就是min（left_max,right_max）- 该点的height；