index.js

// 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.

// Solution

const calPoints = (ops) => {
    const score = [], len = ops.length;

    for (let i = 0; i < len; i++) {
        let point = ops[i];

        if (point === 'D') {
            let last = score[score.length - 1]
            score.push(last * 2)
        }
        else if (point === 'C') {
            score.pop()
        }
        else if (point === '+') {
            let last1 = score[score.length - 1]
            let last2 = score[score.length - 2]
            score.push(last1 + last2)
        } else {
            score.push(+point)
        }
    }
    
    return score.reduce((acc, el) => acc + el)
};