mutual_friends_suggestions.scala

// given
def getFriends(id: Id): Set[Id] = ???

// implement suggestions for friends who have the most mutual friends with user
def suggestedFriends(id: Id, limit: Int): Set[Id] = {
  require(limit > 0)
  require(id != null)
  
  val friends = getFriends(id)
  val friendsOfFriends = 
    friends.toSeq
    .flatMap(f => (getFriends(f).diff(friends) - id).toSeq)
  
  friendsOfFriends
    .groupBy(identity)
    .sortBy(_._2.size)(Ordering[Int].reverse)
    .take(limit)
    .map(_._1)
    .toSet
}