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;
    }
};