# 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)

# Time:  O(n)
# Space: O(n)

# You're now a baseball game point recorder.
# Given a list of strings, each string can be one of the 4 following types:
#
# 1. Integer (one round's score): Directly represents the number of points you get in this round.
# 2. "+" (one round's score): Represents that the points you get in this round are
#                             the sum of the last two valid round's points.
# 3. "D" (one round's score): Represents that the points you get in this round are
#                             the doubled data of the last valid round's points.
# 4. "C" (an operation, which isn't a round's score): Represents the last valid round's points
#                              you get were invalid and should be removed.
#       
# Each round's operation is permanent and could have an impact on the round before and the round after.
# You need to return the sum of the points you could get in all the rounds.
#
# Example 1:
#
# Input: ["5","2","C","D","+"]
# Output: 30
# Explanation: 
# Round 1: You could get 5 points. The sum is: 5.
# Round 2: You could get 2 points. The sum is: 7.
# Operation 1: The round 2's data was invalid. The sum is: 5.  
# Round 3: You could get 10 points (the round 2's data has been removed). The sum is: 15.
# Round 4: You could get 5 + 10 = 15 points. The sum is: 30.
#                                 
# Example 2:
#
# Input: ["5","-2","4","C","D","9","+","+"]
# Output: 27
# Explanation: 
# Round 1: You could get 5 points. The sum is: 5.
# Round 2: You could get -2 points. The sum is: 3.
# Round 3: You could get 4 points. The sum is: 7.
# Operation 1: The round 3's data is invalid. The sum is: 3.  
# Round 4: You could get -4 points (the round 3's data has been removed). The sum is: -1.
# Round 5: You could get 9 points. The sum is: 8.
# Round 6: You could get -4 + 9 = 5 points. The sum is 13.
# Round 7: You could get 9 + 5 = 14 points. The sum is 27.
#
# Note:
# The size of the input list will be between 1 and 1000.
# Every integer represented in the list will be between -30000 and 30000.
                                  
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)

Update baseball-game.py