LeetCode-Baseball Game

Description：

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.

• 如果字元串是由數字組成的，則總分累加上這個數值；
• 如果字元串是“C”，那麼最後一次累加的值無效，需要從總分中減去這部分；
• 如果字元串是“D”，那麼總分累加上最後一個有效數字的兩倍（記為doubleValue），并且這個時候doubleValue成為最後一個有效數值；
• 如果字元串是“+”，那麼總分累加上最後兩個有效數字的和（記為plusValue），并且這個時候plusValue成為最後一個有效數值；

Java

class Solution {
    public int calPoints(String[] ops) {
        int result = 0;
        LinkedList<Integer> list = new LinkedList<>();
        for (String op : ops) {
            int value;
            if (Character.isDigit(op.charAt(0)) || op.charAt(0) == '-') {
                value = Integer.valueOf(op);
                list.addLast(value);
                result += value;
            } else if ("C".equals(op)) {
                value = list.removeLast();
                result -= value;
            } else if ("D".equals(op)) {
                value = list.getLast();
                result += 2 * value;
                list.addLast(2 * value);
            } else if ("+".equals(op)) {
                value = list.getLast() + list.get(list.size() - 2);
                result += value;
                list.addLast(value);
            }
        }
        return result;
    }
}