input: Write a function to calculate the maximum revenue from a list of stock prices. The function should find the maximum profit that can be achieved by buying and selling the stock at different days.
  
  def max_revenue(prices):
      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
  
  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([9, 11, 8, 5, 7, 10])==5
  assert max_revenue([10])==0
output: def max_revenue(prices):
      if len(prices) < 2:
          return 0
      min_price = prices[0]
      max_profit = 0
      for price in prices:
          if price < min_price:
              min_price = price
          else:
              max_profit = max(max_profit, price - min_price)
      return max_profit