Write a method named largestRectangleArea that accepts an array of bar heights as its parameter and finds the largest rectangular area that can be drawn in a contiguous region inside those bars. For example, if the bar heights are {1, 2, 1, 6, 5, 2, 3}, the bars could be thought of as follows:

      #
      # #
      # #
      # #   #
  #   # # # #
# # # # # # #
1 2 1 6 5 2 3
The largest rectangle that can be made within these bars has an area of 10 (2 * 5):

      #
     +---+
     |# #|
     |# #|
     |# #|  #
  #  |# #|# #
# # #|# #|# #
     +---+
1 2 1 6 5 2 3
If the array is empty, return 0. If it contains only a single bar, 
return that bar's height.

The challenge is to solve this problem in linear time.
Brute force will pass the test cases but is too slow.