In Scala, scala.collection.mutable implements Stack data structure. The +: method is similar to ++ method in Stack which gives a copy of the stack with an element prepended. Note that the ending operators are right associative.
Method Definition – def +:(elem: A)
Returns – A new stack consisting of elem followed by all elements of this stack.
Stack(1, for, geeks)
List(List(11, 12, 13, 14, 15), 1)