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5. Longest Palindromic Substring (Medium)

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

 

Example 1:

Input: s = "babad"
Output: "bab"
Note: "aba" is also a valid answer.

Example 2:

Input: s = "cbbd"
Output: "bb"

Example 3:

Input: s = "a"
Output: "a"

Example 4:

Input: s = "ac"
Output: "a"

 

Constraints:

• 1 <= s.length <= 1000
• s consist of only digits and English letters (lower-case and/or upper-case),

Related Topics

[String] [Dynamic Programming]

Similar Questions

1. Shortest Palindrome (Hard)
2. Palindrome Permutation (Easy)
3. Palindrome Pairs (Hard)
4. Longest Palindromic Subsequence (Medium)
5. Palindromic Substrings (Medium)

Hints

Hint 1

How can we reuse a previously computed palindrome to compute a larger palindrome?

Hint 2

If “aba” is a palindrome, is “xabax” a palindrome? Similarly is “xabay” a palindrome?

Hint 3

Complexity based hint:
If we use brute-force and check whether for every start and end position a substring is a palindrome we have O(n^2) start - end pairs and O(n) palindromic checks. Can we reduce the time for palindromic checks to O(1) by reusing some previous computation.