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) {
        stack<int> s;
        int res = 0;
        heights.push_back(0);
        for(int i=0;i<heights.size();){
            if(s.empty() || heights[i] > heights[s.top()]){
                s.push(i++);
            }else{
                int tmp = s.top();
                s.pop();
                if(s.empty()){
                    res = max(res,heights[tmp]*i);
                }else{
                    res = max(res,heights[tmp]*(i-s.top()-1));
                }
            }
        }
        return res;
    }
};