LeetCode 1678 - Goal Parser Interpretation

LeetCode Problem 1678

Difficulty: 🟢 Easy
Topics: String

Solution

Problem Understanding

This problem asks us to interpret a Goal Parser command string. The string command consists of the characters "G", "()", and "(al)" in some order. Each symbol has a specific interpretation: "G" is interpreted as "G", "()" is interpreted as "o", and "(al)" is interpreted as "al". The final result is the concatenation of all interpreted parts in the same order as they appear in the string.

The input is a string of length between 1 and 100. This constraint implies that the input is relatively small, so even an O(n) solution is acceptable. We are guaranteed that the string contains only the specified symbols, so we do not need to handle invalid characters.

Key points to note are that "()" and "(al)" are distinct interpretations despite both containing parentheses, and a naive implementation might confuse one for the other if not carefully checked. We must also handle consecutive occurrences of these patterns correctly, such as "()()()".

Important edge cases include a command consisting only of "G", only "()" repeated many times, only "(al)" repeated many times, and mixed orders.

Approaches

The brute-force approach would scan the string character by character, checking each symbol. Whenever a "G" is found, append "G" to the output. Whenever "(" is encountered, check if the next character is ")" for "()" or check the next three characters for "(al)". This approach is correct and straightforward because it follows the problem statement literally.

A more concise approach leverages string replacement. Since "()" and "(al)" are fixed patterns, we can replace "()" with "o" and "(al)" with "al" in a single pass, then return the final string. This avoids explicit iteration logic and reduces code complexity.

Approach	Time Complexity	Space Complexity	Notes
Brute Force	O(n)	O(n)	Iterates character by character, checks each symbol
Replacement	O(n)	O(n)	Uses built-in string replacement functions for clarity

Algorithm Walkthrough

1. Start with the input string command.
2. Replace all occurrences of "()" with "o". This handles every "()" pattern regardless of position.
3. Replace all occurrences of "(al)" with "al". This handles every "(al)" pattern.
4. Return the resulting string as the interpreted command.

Why it works: The problem guarantees that the input contains only "G", "()", and "(al)". By performing replacements in this order, we ensure that every "()" is correctly interpreted as "o", and every "(al)" is interpreted as "al". The order of replacements does not affect "G", so the resulting string matches the intended Goal Parser interpretation.

Python Solution

class Solution:
    def interpret(self, command: str) -> str:
        result = command.replace("()", "o")
        result = result.replace("(al)", "al")
        return result

The Python implementation uses the str.replace method twice. First, it replaces all "()" with "o", and then it replaces all "(al)" with "al". This implementation is concise and directly follows the algorithm steps above. The resulting string is returned as the final interpretation.

Go Solution

import "strings"

func interpret(command string) string {
    result := strings.ReplaceAll(command, "()", "o")
    result = strings.ReplaceAll(result, "(al)", "al")
    return result
}

The Go implementation uses strings.ReplaceAll to perform the same replacements as the Python solution. strings.ReplaceAll is a safe way to replace all instances of a substring without worrying about overlapping patterns. This implementation handles all valid inputs directly.

Worked Examples

Example 1: "G()(al)"

Step	Current String	Action	Result
1	"G()(al)"	Replace "()" with "o"	"Go(al)"
2	"Go(al)"	Replace "(al)" with "al"	"Goal"

Example 2: "G()()()()(al)"

Step	Current String	Action	Result
1	"G()()()()(al)"	Replace "()" with "o"	"Goooo(al)"
2	"Goooo(al)"	Replace "(al)" with "al"	"Gooooal"

Example 3: "(al)G(al)()()G"

Step	Current String	Action	Result
1	"(al)G(al)()()G"	Replace "()" with "o"	"(al)GalooG"
2	"(al)GalooG"	Replace "(al)" with "al"	"alGalooG"

Complexity Analysis

Measure	Complexity	Explanation
Time	O(n)	Each replacement scans the string once, where n is the length of command.
Space	O(n)	The result string is a new string of length at most n.

The algorithm is efficient for the given input constraints because n <= 100, so two full passes over the string are trivial. Using replacements avoids manual iteration and index tracking.

Test Cases

# provided examples
assert Solution().interpret("G()(al)") == "Goal"  # simple mix
assert Solution().interpret("G()()()()(al)") == "Gooooal"  # multiple "()"
assert Solution().interpret("(al)G(al)()()G") == "alGalooG"  # mixed order

# edge cases
assert Solution().interpret("G") == "G"  # only G
assert Solution().interpret("()") == "o"  # only "()"
assert Solution().interpret("(al)") == "al"  # only "(al)"
assert Solution().interpret("G()G()G") == "GoGoG"  # alternating G and "()"
assert Solution().interpret("(al)(al)(al)") == "alalal"  # multiple "(al)" consecutively
assert Solution().interpret("G()()G(al)(al)()") == "GooGalalo"  # mixed patterns

Test	Why
"G()(al)"	basic mix of all patterns
"G()()()()(al)"	multiple consecutive "()" patterns
"(al)G(al)()()G"	mixed order of all patterns
"G"	only "G" edge case
"()"	only "()" edge case
"(al)"	only "(al)" edge case
"G()G()G"	alternating "G" and "()" patterns
"(al)(al)(al)"	consecutive "(al)" patterns
"G()()G(al)(al)()"	mixed consecutive patterns

Edge Cases

1. Single-character commands: A string like "G" tests the minimal input. The implementation handles this because no replacements are needed; "G" remains "G".
2. Multiple consecutive "()" patterns: A string like "()()()" could confuse an index-based implementation if it does not increment properly. Using string replacement ensures all instances are handled at once without skipping or overlapping.
3. Multiple consecutive "(al)" patterns: A string like "(al)(al)(al)" ensures the parser correctly interprets multiple long patterns in sequence. Our implementation handles this correctly with a single replacement pass for "(al)", guaranteeing the output is "alalal".