Find the longest substring (within a given string) that is the same in reverse.

ReverseSubstring.scala

object ReverseSubstring {

  /**
   * Find the longest substring (within a given string) that is the same in reverse.
   * Example: "I like bananas but not cucumbers." should return "anana".
   */
  def main(args: Array[String]) {
    val biggestSubstring = findBiggestReverseSubstring(args(0))
    println("biggest substring is '%s'".format(biggestSubstring))
  }

  def findBiggestReverseSubstring(text: String): String = {
    val arrayText = text.toArray
    val substrings = (0 until text.length).map(findBiggestReverseSubstringStartingFrom(_, text))
    substrings.max(new Ordering[String] {
      def compare(x: String, y: String): Int = {
        x.length - y.length
      }
    })
  }

  /**
   * Returns the biggest reverse substring by expanding around 'startingPos'.
   */
  def findBiggestReverseSubstringStartingFrom(startingPos: Int, text: Seq[Char]): String = {
    require(startingPos >= 0 && startingPos < text.length, "precondition not met: startingPos is out of bounds")

    def expandReverseSubstring(leftPos: Int, rightPos: Int, text: Seq[Char]): String = {
      require(isReverseSubstring(text.view(leftPos, rightPos + 1)), "precondition not met: current selection is not a reverse substring")

      if (leftPos > 0 && rightPos < text.length - 1 && isReverseSubstring(text.view(leftPos - 1, rightPos + 2)))
        expandReverseSubstring(leftPos - 1, rightPos + 1, text)
      else
        text.slice(leftPos, rightPos + 1).toString
    }

    val oddLengthMaxString = expandReverseSubstring(startingPos, startingPos, text)
    val evenLengthMaxString = expandReverseSubstring(startingPos, startingPos - 1, text)
    if (oddLengthMaxString.length > evenLengthMaxString.length)
      oddLengthMaxString
    else
      evenLengthMaxString

  }

  def isReverseSubstring(text: Seq[Char]): Boolean = {
    val splitIdx = text.length / 2
    text.take(splitIdx) == text.reverse.take(splitIdx)
  }

}

Linear time solution: http://www.akalin.cx/2007/11/28/finding-the-longest-palindromic-substring-in-linear-time/