input: 
  def max_revenue(prices):
      """ Calculate the maximum revenue that can be obtained by buying and selling a stock once.
      The list 'prices' contains the stock prices over a period of time. The goal is to find the
      maximum difference between any two prices where the buying price comes before the selling price.
  
      >>> max_revenue([10, 7, 5, 8, 11, 9])
      6
      >>> max_revenue([1, 2, 3, 4, 5])
      4
      """
      diff = [prices[i + 1] - prices[i] for i in range(len(prices) - 1)] 
      max_diff = 0
      for i in range(len(diff)): 
          for j in range(i + 1, len(diff) + 1): 
              current_diff = sum(diff[i:j]) 
              if current_diff > max_diff: 
                  max_diff = current_diff 
   
      return max_diff
  
output:     min_price = prices[0]
      max_profit = 0
      for price in prices:
          min_price = min(min_price, price)
          max_profit = max(max_profit, price - min_price)
      return max_profit