input: Write a function to calculate the maximum revenue from buying and selling stocks, given an array of prices where the indices represent the days. The function should return the maximum possible profit.
  
  
  def max_revenue(prices):
      if not prices:
          return 0
  
      min_price = prices[0]
      max_profit = 0
  
      for price in prices:
          if price < min_price:
              min_price = price
          elif price - min_price > max_profit:
              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([7, 6, 4, 3, 1]) == 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:
          if price < min_price:
              min_price = price
          elif price - min_price > max_profit:
              max_profit = price - min_price
  
      return max_profit