Goal Parser Interpretation - Solution & Explanation

Problem Overview: You receive a command string used by the Goal Parser. The parser interprets patterns using simple rules: "G" -> "G", "()" -> "o", and "(al)" -> "al". The task is to scan the input command and return the interpreted string after applying these mappings.

This problem is fundamentally a string parsing task. The input contains predictable patterns, so the goal is simply to recognize them efficiently while traversing the string once.

Approach 1: String Replacement (O(n) time, O(n) space)

The most direct solution replaces the known patterns with their interpreted values. Since the command only contains three valid tokens (G, (), and (al)), you can apply sequential string replacements: replace () with o and (al) with al. After replacements, the remaining characters are already valid output.

Another variation scans the string using an index pointer. When you encounter 'G', append it to the result. If you see '(', check the next characters: () becomes "o", while (al) becomes "al". Move the pointer accordingly. Each character is processed once, giving O(n) time complexity and O(n) space for the output string.

This approach works because the grammar is fixed and non-overlapping. There is no ambiguity, so simple conditional checks or replacement operations are sufficient. For interview settings, the pointer-based scan demonstrates clear understanding of string manipulation and avoids unnecessary intermediate copies.

Approach 2: Pattern Matching with Regular Expressions (O(n) time, O(n) space)

You can also solve the problem using regular expressions. Define a pattern that matches \(\) and \(al\), then replace them with their interpreted values. Regex engines efficiently scan the string and apply substitutions in a single pass.

For example, perform two replacements: convert () to "o", then convert (al) to "al". Most modern regex implementations run in linear time for this type of fixed pattern matching, resulting in O(n) time complexity with O(n) additional space for the transformed string.

This approach is concise and expressive, especially in languages like Python or JavaScript where regex replacements are built-in. However, interviewers often prefer explicit parsing logic because it demonstrates control over string traversal and avoids reliance on regex libraries.

Recommended for interviews: Use the pointer-based string scan from the String Replacement approach. It processes the input once in O(n) time with O(n) space, clearly shows how you identify tokens like () and (al), and avoids unnecessary library calls. The regex approach is shorter but hides the parsing logic, which interviewers usually want to see.