gistfile1.scala

import scala.collection.mutable.ListBuffer
import scala.collection.mutable.StringBuilder

object Main {

  def main(args: Array[String]) {

    try {
      println(searchLongestSubsequence(""))
    } catch {
      case e: Exception => println(e.toString())           // "Empty String"
    }

    println(searchLongestSubsequence("010"))               // 01
    println(searchLongestSubsequence("1123"))              // 123
    println(searchLongestSubsequence("122345"))            // 2345
    println(searchLongestSubsequence("987123"))            // 123
    println(searchLongestSubsequence("97463288890"))       // 46
    println(searchLongestSubsequence("83625469835377789")) // 469

  }

  def searchLongestSubsequence(str: String) = {

    def splitIncreasingSubsequence(str: String): List[String] = {
      val subseqs = new ListBuffer[String]
      val subseq  = new ListBuffer[Char]

      def split(chars: List[Char]) {
        chars match {
          case hd :: Nil => {
            subseq += hd
            subseqs += subseq.mkString
          }
          case hd :: tl if (hd < tl.head) => {
            subseq += hd
            split(tl)
          }
          case hd :: tl if (hd >= tl.head) => {
            subseq += hd
            subseqs += subseq.mkString
            subseq.clear()
            split(tl)
          }
        }
      }

      split(str.toList)
      subseqs.toList
    }

    str match {
      case "" => throw new IllegalArgumentException("Empty String.")
      case xs => splitIncreasingSubsequence(xs).maxBy(_.length)
    }
  }
}