0005.Longest Palindromic Substring

Link: 5. Longest Palindromic Substring - Medium

Track: NeetCode150

Question

Restate the problem

Given a string of lowercase English letters.
Return the longest contiguous substring that is palindrome.

• palindromic: it reads the same forwards and backwards. mirrors around its center.

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• Method 1: Brute Force, check every possible substring. take O($n^3$)
• Method 2: Expand around the center

Method: Expand around the center

Approach

Every palindrome can be expanded from its center.

• Iterate through each index, consider two cases: odd, even
• Use two pointers to expand outward while characters match
• After expansion stops, restore two pointers to get validate palindrome range.
• Track the maximum length and record the substring

Complexity

• Time Complexity: O($n^2$)
• Space Complexity: O(1) (excluding output)

Edge Case

• Empty string → return ""

Code

class Solution:
    def longestPalindrome(self, s: str) -> str:
        n = len(s)
        max_length = 0
        res = ""
 
        def expand(left, right):
            while 0 <= left and right < n and s[left] == s[right]:
                left -= 1
                right += 1
            
            return (left + 1, right - 1)
 
        
        for idx in range(n):
            even_left, even_right = expand(idx, idx + 1)
            odd_left, odd_right = expand(idx, idx)
 
            if max_length < even_right - even_left + 1:
                max_length = even_right - even_left + 1
                res = s[even_left: even_right + 1]
            
            if max_length < odd_right - odd_left + 1:
                max_length = odd_right - odd_left + 1
                res = s[odd_left: odd_right + 1]
 
        return res
 

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History

Jan-25-2026 Solved w bug
Jan-22-2026 Solved w bug

• return indentation

Jan-22-2026 Solved w bug

• Wrote the odd_pointer update incorrectly.

Jan-19-2026 Solved w bug, forget to restore pointer