input: Write a function to calculate the maximum revenue by finding the maximum price difference between any two prices in the given list. If the list is empty, return 0.
  
  assert max_revenue([10, 7, 5, 8, 11, 9])==6
  assert max_revenue([1, 2, 3, 4, 5])==4
  assert max_revenue([5, 4, 3, 2, 1])==0
  assert max_revenue([10])==0
  assert max_revenue([])==0
output: def max_revenue(prices):
      if not prices:
          return 0
      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