Goal Parser Interpretation

Goal Parser Interpretation

What is this problem about?

The Goal Parser Interpretation interview question is a basic string parsing task. You are given a string command that consists of the string "G", "()", and "(al)" in some order. The parser needs to interpret:

• "G" as the string "G".
• "()" as the string "o".
• "(al)" as the string "al". Your goal is to return the fully interpreted string.

Why is this asked in interviews?

Google uses this String coding problem as a very quick initial screening question. It tests your basic programming syntax, specifically string replacement or simple loop iteration. It’s an "Easy" problem designed to ensure a candidate can write a functioning block of code quickly and without syntax errors.

Algorithmic pattern used

This problem can be solved using String Replacement or a Linear Scan.

1. String Replacement: Use the built-in string replace function of your language. Replace "()" with "o", and "(al)" with "al".
2. Linear Scan: Iterate through the string.
   • If command[i] == 'G', append 'G'.
   • If command[i] == '(':
     • If command[i+1] == ')', append 'o' and skip to $i+2$.
     • If command[i+1] == 'a', append 'al' and skip to $i+4$.

Example explanation

Command: "G()(al)"

1. Using Linear Scan:
   • i=0: 'G'. Append 'G'.
   • i=1: '('. Check next char. i+1 is ')'. Append 'o'. Advance i by 2.
   • i=3: '('. Check next char. i+1 is 'a'. Append 'al'. Advance i by 4. Result: "Goal".

Common mistakes candidates make

• Index Out of Bounds: When checking command[i+1] during a manual linear scan, forgetting to ensure that i+1 is within the bounds of the string (though valid inputs here prevent it, it's a bad practice).
• String immutability: In languages like Java, doing result += char in a loop creates many temporary objects. Using a StringBuilder or the built-in .replace() is better.
• Over-engineering: Using complex Regular Expressions for something that simple exact-string replacements can handle faster and more readably.

Interview preparation tip

While .replace("()", "o").replace("(al)", "al") is a one-liner and perfectly valid, be prepared to write the loop-based parser if the interviewer asks you to optimize it for a single pass without allocating intermediate strings.