leetcode 42: Trapping Rain Water

class Solution {
public:
    int trap(vector<int>& height) {
        int begin;
        int res=0;
        for(begin=0;begin<height.size();begin++)//find the first non-zero position
            if(height[begin]!=0)
                break;
        while(begin<height.size())
        {
            int end,max_idx=begin+1,max=0;
            for(end=begin+1;end<height.size();end++)//find the end position of the container
            {
                if(height[end]>=height[begin])
                {
                    break;
                }
                if(height[end]>max)//mark the position with the maximum height
                {
                    max=height[end];
                    max_idx=end;
                }
                res+=height[begin]-height[end];
            }
            if(end!=height.size())//exists the end position with the height greater than or equal to the begin position
                begin=end;
            else//end height is smaller than the begin position, subtract the heights after the max_idx and the redundant height before it
            {
                for(int i=max_idx;i<end;i++)
                    res-=height[begin]-height[i];
                res-=(max_idx-begin-1)*(height[begin]-max);
                begin=max_idx;
            }
        }
        return res;
    }
};