Largest Rectangle in Histogram

Given n non-negative integers representing the histogram's bar height where the width of each bar is 1, find the area of largest rectangle in the histogram.

Above is a histogram where width of each bar is 1, given height = [2,1,5,6,2,3].

The largest rectangle is shown in the shaded area, which has area = 10 unit.

For example,
Given heights = [2,1,5,6,2,3],
return 10.

class Solution {
public:
    int largestRectangleArea(vector<int>& heights) {
        
       if( heights.size() == 0)
           return 0;
        
       
        heights.push_back(-1);
        int max = 0;
        int index = 0;
        stack<int> s;
        while(index < heights.size())
        {
            if(s.size() == 0 || heights[s.top()] <= heights[index])
            {
                s.push(index);
                index++;
            }
            else
            {
                int top = s.top();
                s.pop();
                int size = 0;
                if(s.size() == 0)
                    size = heights[top]*index;
                else
                    size = heights[top]*(index - s.top() -1);
                
                
                if(size > max)
                    max = size;
            }
        }
        
        return max;
        
        
        
        
    }
};

终于写出来了，唉