Challenges.scala

object Challenges {
  def mergeStrings(a: String, b: String): String = {
    a.zipAll(b, "", "") map { case (m, n) => m.toString + n } mkString
  }
  
  def mergeStringsRec(a: String, b: String): String = {
    def loop(first: List[Char], second: List[Char]): String =
      (first, second) match {
        case (x :: xs, y :: ys) => x.toString + y + loop(xs, ys)
        case (f, Nil) => f mkString
        case (Nil, s) => s mkString
      }
    loop(a.toList, b.toList)
  }

  def electionWinner(votes: Array[String]): String = {
    val countedVotes = votes.groupBy(identity).mapValues(_.length)
    val maxVotes = countedVotes.maxBy(_._2)._2
    val allWinner = countedVotes.filter {
      case (_, count) => count == maxVotes
    }
    allWinner.keys.toList.max // select alphabetically last
  }
}

ChallengesTest.scala

import org.scalatest.{FunSuite, Matchers}

class ChallengesTest extends FunSuite with Matchers {
  test("merge empty") {
    Challenges.mergeStrings("", "") should be("")
  }

  test("merge equal length") {
    Challenges.mergeStrings("aaa", "bbb") should be("ababab")
  }

  test("merge a string longer") {
    Challenges.mergeStrings("abcdedg", "???") should be("a?b?c?dedg")
  }

  test("merge b string longer") {
    Challenges.mergeStrings("HTML", "........") should be("H.T.M.L.....")
  }

  test("ballot of one") {
    val votes = Array("Alice")
    Challenges.electionWinner(votes) should be("Alice")
  }

  test("ballot with clear winner") {
    val votes = Array("John", "John", "John", "Bob", "John", "Alice")
    Challenges.electionWinner(votes) should be("John")
  }

  test("ballot with more than one with max votes") {
    val votes = Array("Alex",
                      "Michael",
                      "Harry",
                      "Dave",
                      "Michael",
                      "Victor",
                      "Harry",
                      "Alex",
                      "Mary",
                      "Mary")
    Challenges.electionWinner(votes) should be("Michael")
  }
}