leetcode 84 柱状图中最大的矩形

class Solution {
public:
    int largestRectangleArea(vector<int>& heights) {
        stack<int> s;
        int res=0, i=0, size=heights.size();
        int left=-1, right=size, val,s_top;
        //
        while(i<size){
            if(!s.empty() && heights[i]<heights[s.top()]){
                while(!s.empty()&&heights[i]<heights[s.top()]){
                    s_top=s.top();
                    val=heights[s_top];
                    right = i-s_top;//该长条距离右边i的距离
                    s.pop();
                    //该长条距离左边比它长度小或-1的距离
                    left=s.empty()?s_top+1:s_top-s.top();
                    //左边长加右边长减一 再乘以该长条高度
                    res=max(val*(right+left-1),res);
                }
            }
            s.push(i);
            i++;
        }
        i--;//因为此时i==heights.size(); 所以要减减
        //处理栈中还存在的递增序列
        while(!s.empty()){
            right=size-i;
            val =heights[i];
            s.pop();
            left=s.empty()?i+1:i-s.top();
            res=max(val*(right+left-1),res);
            if(s.empty())
                return res;
            i=s.top();
        }
        return res;
    }
};