input: Write a function to calculate the maximum revenue that can be obtained from a given list of prices by buying and selling stocks on different days. The function should return the maximum possible revenue. If the given list of prices is empty, the function should return 0.
  
  def max_revenue(prices):
      if not prices:
          return 0
      
      max_profit = 0
      min_price = prices[0]
      
      for price in prices:
          min_price = min(min_price, price)
          max_profit = max(max_profit, price - min_price)
      
      return max_profit
  
  assert max_revenue([10, 7, 5, 8, 11, 9]) == 6
  assert max_revenue([7, 1, 5, 3, 6, 4]) == 5
  assert max_revenue([1, 2, 3, 4, 5]) == 4
  assert max_revenue([7, 6, 4, 3, 1]) == 0
  assert max_revenue([]) == 0
output: def max_revenue(prices):
      if not prices:
          return 0
      
      max_profit = 0
      min_price = prices[0]
      
      for price in prices:
          min_price = min(min_price, price)
          max_profit = max(max_profit, price - min_price)
      
      return max_profit