Functional Programming: Does a list only contain unique items?

Question

I\'m having an unsorted list and want to know, whether all items in it are unique.
My naive approach would be

val l = List(1,2,3,4,3)
def isUniqueList(l: List[Int         


        

Answer 1

Here is the fastest purely functional solution I can think of:

def isUniqueList(l: List[T]) = isUniqueList1(l, new HashSet[T])

@tailrec
def isUniqueList1(l: List[T], s: Set[T]) = l match {
  case Nil => true
  case (h :: t) => if (s(h)) false else isUniqueList1(t, s + h)
}

This should be faster, but uses mutable data structure (based on the distinct implementation given by Vasil Remeniuk):

def isUniqueList(l: List[T]): Boolean = {
  val seen = mutable.HashSet[A]()
  for (x <- this) {
    if (seen(x)) {
      return false
    }
    else {
      seen += x
    }
  }
  true
}

And here is the simplest (equivalent to yours):

def isUniqueList(l: List[T]) = l.toSet.size == l.size