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 twovalidround's points.
3. "D"(one round's score): Represents that the points you get in this round are the doubled data of the lastvalidround's points.
4. "C"(an operation, which isn't a round's score): Represents the lastvalidround'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.

用一个stack记录之前的数字

class Solution {
    public int calPoints(String[] ops) {
        if(ops == null || ops.length == 0)
            return 0;

        Stack<Integer> stack = new Stack<Integer>();

        for(String op: ops) {
            if(op.equals("C")) {
                stack.pop();
            } else if(op.equals("D")) {
                stack.push(stack.peek() * 2);
            } else if(op.equals("+")) {
                int num2 = stack.pop();
                int num1 = stack.peek();
                stack.push(num2);
                stack.push(num1 + num2);
            } else {
                stack.push(Integer.parseInt(op));
            }
        }

        int res = 0;
        while(stack.size() > 0) {
            res += stack.pop();
        }

        return res;
    }
}