'''https://www.lintcode.com/problem/largest-rectangle-in-histogram/description
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.

histogram

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

histogram

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

Example
Given height = [2,1,5,6,2,3],
return 10.
'''