Longest Palindromic Substring

Problem

Given a string s, return the longest palindromic substring in s.

Examples

The brute-force approach

Try every substring. Check if it's a palindrome. Track the longest.

best = ""
for i in range(n):
    for j in range(i, n):
        substr = s[i:j+1]
        if is_palindrome(substr) and len(substr) > len(best):
            best = substr
return best

O(n³) — O(n²) substrings each requiring O(n) palindrome check.

Spotting the pattern

This is a Two Pointers problem. The key question to ask yourself:

Why do you need to check both odd-length and even-length palindrome centers separately? What's the center of "aba" vs "abba"?

Answering that is where it clicks, and it's exactly what the guided walkthrough below builds with you: the pattern reasoning, a progressive hint ladder that never spoils the answer, a row-by-row dry run, the optimized solution, and an in-browser editor to run your code against real test cases.