leetcode 84 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.

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class Solution {public:    int largestRectangleArea(vector<int>& heights) {        vector<int> stack;int len = heights.size();if(len == 0) return 0;int index = 0, max_area = 0;heights.push_back(-1);while (index <= len) {if(stack.empty() || heights[index] >= heights[stack.back()]) {stack.push_back(index);index++;} else {int top = stack.back(), temp = 0;stack.pop_back();if(stack.empty()) {temp = heights[top]*index;} else {temp = heights[top]*(index-stack.back()-1);}if(max_area < temp) max_area = temp;}}return max_area;    }};