Python – Longest Palindromic Substring: Center Expansion vs One-Liner (Trade-Offs) #CodingInterview

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The Longest Palindromic Substring problem is a classic interview question where you must find the longest substring in a given string that reads the same backward as forward.

Method 1 – Center Expansion (Traditional Approach):

How It Works:

This method iterates over each character in the string, treating it as the center of a possible palindrome. It then expands outward for both odd-length (single center) and even-length (dual center) palindromes. By continuously updating the longest palindrome found, it provides a reliable solution.

Efficiency:

Time Complexity: O(n²), where n is the length of the string.

Space Complexity: O(1) – It only uses a few extra pointers.

Advantages:

Better suited for long strings.

More robust and easier to handle edge cases.

Method 2 – One-Liner Approach:

How It Works:

This concise solution uses a generator expression to produce all possible substrings that are palindromes by checking each substring against its reverse. It then selects the longest one using Python’s built-in max() function.

Efficiency:

Time Complexity: O(n³) in the worst case due to generating and checking all substrings.

Space Complexity: O(n²), as many substrings may be generated.

Trade-Offs:

While the one-liner is neat and expressive, it is less efficient and may become problematic with very long strings.

It is best used for small inputs or quick demonstrations where brevity is preferred over performance.

Why Is This Problem Important?

Interview Relevance:

It tests your understanding of string manipulation, dynamic programming, and algorithm optimization—common topics in technical interviews.

Practical Applications:

Useful in text processing, bioinformatics (DNA sequence analysis), and any domain where identifying symmetric patterns is needed.

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