Longest Palindromic Substring

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.

Scala

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")

Second Tab