Medium — String | Two Pointers | Expand Around Centers | Palindrome

The Problem

Given a string, find and return the longest contiguous substring that reads the same forwards and backwards (palindrome).

Approach

Use the expand around centers technique by checking each possible center position in the string. For each center, expand outwards while characters match to find the longest palindrome, considering both odd-length (single center) and even-length (two adjacent centers) palindromes.

Time: O(n²) · Space: O(1)

Code

class Solution:
    def longestPalindrome(self, s: str) -> str:
        def expandAroundCenter(left: int, right: int) -> str:
            # Expand around the center while the characters at both ends are equal.
            while left >= 0 and right < len(s) and s[left] == s[right]:
                left -= 1
                right += 1
            # Return the palindrome substring.
            return s[left + 1 : right]

        if len(s) < 2:
            return s

        longest = ""

        for i in range(len(s)):
            # Check for odd-length palindromes.
            palindrome1 = expandAroundCenter(i, i)
            if len(palindrome1) > len(longest):
                longest = palindrome1

            # Check for even-length palindromes.
            palindrome2 = expandAroundCenter(i, i + 1)
            if len(palindrome2) > len(longest):
                longest = palindrome2

        return longest

Watch It Run

Try it yourself: Open TraceLit and step through every line.

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