input: Write a function to calculate the maximum revenue that can be obtained by buying and selling stocks at different prices. If the given list of prices is empty, return 0.
  
  def max_revenue(prices):
      if not prices:
          return 0
      max_profit = 0
      min_price = prices[0]
      for price in prices[1:]:
          if price < min_price:
              min_price = price
          else:
              max_profit = max(max_profit, price - min_price)
      return max_profit
  
  assert max_revenue([10, 7, 5, 8, 11, 9])==6
  assert max_revenue([1, 6, 11, 3, 5])==10
  assert max_revenue([])==0
  assert max_revenue([7, 6, 4, 3, 1])==0
  assert max_revenue([7, 7, 7, 7])==0
output: def max_revenue(prices):
      if not prices:
          return 0
      max_profit = 0
      min_price = prices[0]
      for price in prices[1:]:
          if price < min_price:
              min_price = price
          else:
              max_profit = max(max_profit, price - min_price)
      return max_profit