Given a string S, find the longest palindromic substring in S.

You may assume that the maximum length of S is 1000, and there exists one unique longest palindromic substring.

import scala.annotation.tailrec /** * find longest palindrome (length only) * * @param s * @return */ def longestPalindromeLength(s: String): Int = { @tailrec def check(prev: Int, next: Int, acc: Int): Int = { if (prev < 0 || next >= s.length || s(prev) != s(next)) acc else check(prev - 1, next + 1, acc + 2) } var maxLength = 0 for (i <- 0 until s.length) { // check odd length and even length maxLength = maxLength max check(i, i, -1) max check(i, i + 1, 0) } maxLength } longestPalindromeLength("") longestPalindromeLength("a") longestPalindromeLength("bab") longestPalindromeLength("baab") /** * find longest palindrome (substring) * * @param s * @return */ def longestPalindrome(s: String): String = { @tailrec def check(prev: Int, next: Int, acc: Int): Int = { if (prev < 0 || next >= s.length || s(prev) != s(next)) acc else check(prev - 1, next + 1, acc + 2) } var maxLength, length, index = 0 for (i <- 0 until s.length) { length = check(i, i, -1) max check(i, i + 1, 0) maxLength = maxLength max length if (length == maxLength) index = i } // recover string from index and length val diff = maxLength / 2 maxLength match { case l if l % 2 == 0 => s.subSequence(index - diff + 1, index + diff + 1).toString case _ => s.subSequence(index - diff, index + diff + 1).toString } } longestPalindrome("bananas") longestPalindrome("abracadabra")