682 - Baseball game

The problem statement

You are keeping the scores for a baseball game with strange rules. At the beginning of the game, you start with an empty record.

You are given a list of strings operations, where operations[i] is the ith operation you must apply to the record and is one of the following:

• An integer x.
  • Record a new score of x.
• '+'.
  • Record a new score that is the sum of the previous two scores.
• 'D'.
  • Record a new score that is the double of the previous score.
• 'C'.
  • Invalidate the previous score, removing it from the record.

Return the sum of all the scores on the record after applying all the operations.

The test cases are generated such that the answer and all intermediate calculations fit in a 32-bit integer and that all operations are valid.

Understanding the problem

This problem is asking us to essentially perform the basic operations of interacting with a stack.

The instructions are pretty clear. Every test case that they run our code against is going to be valid as specified. So we don't need to create fault tolerant code.

So lets digest:

We want to store a stack of results which are numerical based upon each iteration of operations.

• When x then we want to push that int value into the stack.
• When + then we want to add stack[i-1] to stack[i-2] and push that int value into the stack.
• When D we want to stack[i - 1] * 2 and push that int value into the stack.
• When C we want to pop the last value from the stack.

Then at the end it wants us to return the sum of stack.

That's it. Pretty simple. Let's dive into the code!

Examples:

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","C"] Output: 0 Explanation:

• "1" - Add 1 to the record, record is now [1].
• "C" - Invalidate and remove the previous score, record is now [].
• Since the record is empty, the total sum is 0.

Code solution

function calculatePoints(operations: string[]): number {
    const scores: number[] = [];
    for(let i = 0; i < operations.length; i++) {
        switch(operations[i]) {
            case: '+':
                scores.push(scores[scores.length - 1] + scores[scores.length - 2]);
                break;
            case 'D':
                scores.push(scores[scores.length - 1] * 2);
                break;
            case 'C':
                scores.pop();
                break;
            default:
                scores.push(parseInt(opreations[i]));
                break;
        }
    }
    return scores.reduce((acc, curr) => acc + curr, 0);
}

Key Takeaways

1. Think of arrays like stacks. A stack is just a vertical array with the same concept of LIFO (Last in First out) via it's push/pop mechanism.