input: Write a function to calculate the maximum revenue from a list of prices where you can buy and sell stocks in the same day. If the list is empty, return 0.
  def max_revenue(prices):
      if not prices: 
          return 0
      max_diff = 0
      for i in range(len(prices)): 
          for j in range(i, len(prices)): 
              max_diff = max(max_diff, prices[j] - prices[i]) 
   
      return max_diff
  
  assert max_revenue([10, 7, 5, 8, 11, 9])==6
  assert max_revenue([5, 3, 2])==0
  assert max_revenue([1, 2, 3, 4, 5])==4
  assert max_revenue([])==0
output: def max_revenue(prices):
      if not prices: 
          return 0
      max_diff = 0
      for i in range(len(prices)): 
          for j in range(i, len(prices)): 
              max_diff = max(max_diff, prices[j] - prices[i]) 
   
      return max_diff