BaseballGame682.java

/**
 * You're now a baseball game point recorder.
 * 
 * Given a list of strings, each string can be one of the 4 following types:
 * 
 * Integer (one round's score): Directly represents the number of points you get in this round.
 * "+" (one round's score): Represents that the points you get in this round are the sum of the last two valid round's points.
 * "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.
 * "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.
 */

public class BaseballGame682 {
    public int calPoints(String[] ops) {
        if (ops == null || ops.length == 0) return 0;
        Stack<Integer> stack = new Stack<>();
        int sum = 0;
        for (String op: ops) {
            if (op.equals("+")) {
                if (stack.size() >= 2) {
                    int e1 = stack.pop();
                    int e2 = stack.peek();
                    int twoSum = e1 + e2;
                    sum += twoSum;
                    stack.push(e1);
                    stack.push(twoSum);
                }
            } else if (op.equals("D")) {
                if (!stack.isEmpty()) {
                    int doubledScore = stack.peek() * 2;
                    sum += doubledScore;
                    stack.push(doubledScore);
                }
            } else if (op.equals("C")) {
                if (!stack.isEmpty()) {
                    int lastScore = stack.pop();
                    sum -= lastScore;
                }
            } else {
                int now = Integer.parseInt(op);
                sum += now;
                stack.push(now);
            }
        }
        
        return sum;
    }

}