input: 
  def max_revenue(prices):
      """ Calculate the maximum revenue that can be earned by buying and selling a stock
      on different days. The input is a list of stock prices where the index represents the day.
      
      >>> max_revenue([10, 7, 5, 8, 11, 9])
      6
      >>> max_revenue([100, 180, 260, 310, 40, 535, 695])
      655
      """
      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
  
output:     diff = [prices[i + 1] - prices[i] for i in range(len(prices) - 1)] 
      max_diff = 0
      current_diff = 0
      for d in diff:
          current_diff = max(0, current_diff + d)
          max_diff = max(max_diff, current_diff)
   
      return max_diff