new_contents: # Time:  O(n)
  # Space: O(n)
  
  class Solution(object):
      def calPoints(self, ops):
          """
          :type ops: List[str]
          :rtype: int
          """
          history = []
          for op in ops:
              if op == '+':
                  history.append(history[-1] + history[-2])
              elif op == 'D':
                  history.append(history[-1] * 2)
              elif op == 'C':
                  history.pop()
              else:
                  history.append(int(op))
          return sum(history)
  
subject: Create baseball-game.py