algorithm notes: leetcode#682 baseball game

Problem

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.

Solution

Analysis

Use history to record scores we get in each round, ans to store the sum we should return, and take increment as the number of points we should add to the sum in each round.

Traverse each string in the given list, calculate the increment and add it to ans. In addition, if the string is 'C', we should remove the last score in the history, otherwise, we should append points we get in the current round to history.

Python implementation

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class :
    def calPoints(self, ops):
        """
        :type ops: List[str]
        :rtype: int
        """
        ans = 0
        history = []
        
        for op in ops:
            if op == "+":
                increment = history[-1] + history[-2]
                history.append(increment)
            elif op == "D":
                increment = history[-1] * 2
                history.append(increment)
            elif op =="C":
                increment = - history.pop()
            else:
                increment = int(op)
                history.append(increment)
                
            ans += increment
        
        return ans

Java implementation

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class  {
    public int calPoints(String[] ops) {
        int ans = 0;
        int increment = 0;
        Stack<Integer> history = new Stack();
        
        for(String op : ops){
            if(op.equals("+")){
                int lastOne = history.pop();
                int lastTwo = history.peek();
                increment = lastOne + lastTwo;
                history.push(lastOne);
                history.push(increment);
            }else if(op.equals("D")){
                increment = history.peek() * 2;
                history.push(increment);
            }else if(op.equals("C")){
                increment = -history.pop();
            }else {
                increment = Integer.parseInt(op);
                history.push(increment);
            }
            
            ans += increment;
        }
        
        return ans;        
    }
}

Time complexity

O(n). n is length of the given list, as we iterate it once.

Space complexity

O(n). n is length of the given list, because we need to record points we get in each round.

Links

682. Baseball Game
(中文版) 算法笔记: 力扣#682 棒球比赛