Calculate Score After Performing Instructions

Calculate Score After Performing Instructions

What is this problem about?

The Calculate Score After Performing Instructions interview question involves a simulation of a game or process where you follow a set of rules to update a score. You are typically given an array of initial values and a sequence of commands (like "Double", "Add", "Cancel"). Each command affects the current score or modifies the history of scores. This Calculate Score After Performing Instructions coding problem is a test of data management and rule following.

Why is this asked in interviews?

Apple uses this to test a candidate's ability to maintain a state history. It evaluates whether you can handle "undo" operations (canceling the previous move) or operations that depend on previous results. It's a standard test for basic simulation logic and data structure usage (like stacks or arrays).

Algorithmic pattern used

This utilizes the Array, Hash Table, String, Simulation interview pattern. Often, a Stack is the most intuitive structure because many instructions refer to the "most recent" score. When you receive a "Double" instruction, you look at the top of the stack; when you receive a "Cancel", you pop from the stack.

Example explanation

Instructions: [5, 2, "C", "D", "+"]

1. 5: Push 5. Stack: [5]
2. 2: Push 2. Stack: [5, 2]
3. "C" (Cancel): Pop 2. Stack: [5]
4. "D" (Double): Double top (5) and push 10. Stack: [5, 10]
5. "+" (Add): Sum last two (5+10) and push 15. Stack: [5, 10, 15] Total Score = 5 + 10 + 15 = 30.

Common mistakes candidates make

• Handling "Cancel" on empty history: Not checking if there's a score to cancel or double, which can lead to runtime errors.
• String comparison: Using == instead of .equals() in languages like Java when checking for operation strings.
• Total vs. Local: Confusion between returning the final score added vs. the cumulative sum of all valid scores.

Interview preparation tip

When instructions involve "last", "previous", or "undo", always consider using a Stack. It simplifies the logic by giving you O(1) access to the most recent elements while naturally handling the removal of those elements.