text: """This problem was asked by Facebook.
  
  Given a array of numbers representing the stock prices of 
  a company in chronological order, write a function that calculates 
  the maximum profit you could have made from buying 
  and selling that stock once. You must buy before you can sell it.
  
  For example, given [9, 11, 8, 5, 7, 10], you should return 5, since 
  you could buy the stock at 5 dollars and sell it at 10 dollars.
  """