Leetcode – Goal Parser Interpretation Solution

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String manipulation is an essential skill for any programmer, and coding challenges offer a great way to hone this skill. The “Goal Parser Interpretation” problem on Leetcode is a beginner-friendly problem that tests your grasp of string parsing in Python. Although the problem might seem simple at first glance, it provides an excellent opportunity to understand Python string methods better and discuss various approaches to solve it. This article aims to be a complete guide to tackling this problem, explaining multiple methods, their time and space complexities, and how to improve them.

Problem Statement

You own a Goal Parser that can interpret a string command. The command consists of an alphabet of "G", "()" and/or "(al)" in some order. The Goal Parser will interpret "G" as the string "G", "()" as the string "o", and "(al)" as the string "al". The interpreted strings are then concatenated in the original order.

Given the string command, return the Goal Parser’s interpretation of command.


Input: command = "G()(al)"
Output: "Goal"

Basic Approach: String Replacement

The most straightforward way to tackle this problem is by using Python’s str.replace() method, which replaces occurrences of a substring within a string.

Python Code

def interpret(command):
    return command.replace('()', 'o').replace('(al)', 'al')

Time Complexity

The time complexity for this approach is O(n), where n is the length of the input string. Python’s str.replace() method generally works in linear time.

Optimized Approach: One-pass String Building

Although the previous approach works well, it involves multiple passes over the string. An optimized method would be to build the output string in a single pass.

Python Code

def interpret(command):
    res = []
    i = 0
    while i < len(command):
        if command[i] == 'G':
            i += 1
        elif command[i:i+2] == '()':
            i += 2
            i += 4
    return ''.join(res)

Time Complexity

The time complexity for this approach is O(n), but it makes a single pass over the string, which could be more efficient for long strings.

Testing Your Solution

Before submitting your code on Leetcode, you should test it on multiple test cases to ensure its correctness.

assert interpret("G()(al)") == "Goal"
assert interpret("G()()()()(al)") == "Gooooal"
assert interpret("(al)G(al)()()G") == "alGalooG"
assert interpret("") == ""
assert interpret("G") == "G"


The “Goal Parser Interpretation” problem may seem trivial but offers an excellent opportunity to delve into string manipulation techniques in Python. Whether you are using Python’s built-in str.replace() method or creating a custom function to interpret the string, understanding the underlying principles is crucial for mastering string manipulation tasks.

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