[leetcode #682] Baseball Game

Problem

You are keeping score for a baseball game with strange rules. The game consists of several rounds, where the scores of past rounds may affect future rounds' scores.
At the beginning of the game, you start with an empty record. You are given a list of strings ops, where ops[i] is the ith operation you must apply to the record and is one of the following:

1. An integer x - Record a new score of x.
2. "+" - Record a new score that is the sum of the previous two scores. It is guaranteed there will always be two previous scores.
3. "D" - Record a new score that is double the previous score. It is guaranteed there will always be a previous score.
4. "C" - Invalidate the previous score, removing it from the record. It is guaranteed there will always be a previous score.

Return the sum of all the scores on the record. The test cases are generated so that the answer fits in a 32-bit integer.
Example 1:

Input: ops = ["5","2","C","D","+"]
Output: 30
Explanation:
"5" - Add 5 to the record, record is now [5].
"2" - Add 2 to the record, record is now [5, 2].
"C" - Invalidate and remove the previous score, record is now [5].
"D" - Add 2 * 5 = 10 to the record, record is now [5, 10].
"+" - Add 5 + 10 = 15 to the record, record is now [5, 10, 15].
The total sum is 5 + 10 + 15 = 30.

Example 2:

Input: ops = ["5","-2","4","C","D","9","+","+"]
Output: 27
Explanation:
"5" - Add 5 to the record, record is now [5].
"-2" - Add -2 to the record, record is now [5, -2].
"4" - Add 4 to the record, record is now [5, -2, 4].
"C" - Invalidate and remove the previous score, record is now [5, -2].
"D" - Add 2 * -2 = -4 to the record, record is now [5, -2, -4].
"9" - Add 9 to the record, record is now [5, -2, -4, 9].
"+" - Add -4 + 9 = 5 to the record, record is now [5, -2, -4, 9, 5].
"+" - Add 9 + 5 = 14 to the record, record is now [5, -2, -4, 9, 5, 14].
The total sum is 5 + -2 + -4 + 9 + 5 + 14 = 27.

Example 3:

Input: ops = ["1"]
Output: 1

Constraints:

· 1 <= ops.length <= 1000
· ops[i] is "C", "D", "+", or a string representing an integer in the range [-3 * 10⁴, 3 * 10⁴].
· For operation "+", there will always be at least two previous scores on the record.
· For operations "C" and "D", there will always be at least one previous score on the record.

Idea

これは,規定に従ってゲームを行う場合,最終値がどの程度であるかという問題である.
「+」が表示された場合、前の2つのスコアを加算した値が現在のラウンドのスコアになります.
「D」が出た場合、以前の点数は現在のラウンドの2倍になる.
「C」が表示された場合、以前のスコアは除外されます.
上記の規則に従ってスコアを計算するには、前のラウンドのスコアが必要であるため、stackを使用することが望ましい.条件に応じてスタックの値を用いて点数を減算した後,最終計算の総和を戻せばよい.

Solution

class Solution {
    public int calPoints(String[] ops) {
        Stack<Integer> stack = new Stack<>();
        int ans = 0;

        int tmp = 0;
        int val = 0;
        for (String s : ops) {
            switch(s) {
                case "+":
                    tmp = stack.pop();
                    val = tmp + stack.peek();
                    stack.push(tmp);
                    stack.push(val);
                    ans += val;
                    break;
                case "D":
                    val =  stack.peek() * 2;
                    stack.push(val);
                    ans += val;
                    break;
                case "C":
                    ans -= stack.pop();
                    break;
                default:
                    val = Integer.parseInt(s);
                    stack.push(val);
                    ans += val;
                    break;
            }
        }

        return ans;
    }
}

Reference

https://leetcode.com/problems/baseball-game/