SmallestRange.scala

import collection.immutable.TreeSet
object Solution {
    case class Item(cur: Int, next: List[Int], id: Int)
    implicit val itemOrdering: Ordering[Item] = Ordering.by(i => (i.cur,i.next.headOption, i.id))
    def go(s: TreeSet[Item]): LazyList[Vector[Int]] = 
       Vector(s.head.cur, s.last.cur) #:: (s.head match {
           case Item(_, x :: rest, id) => go(s.tail + Item(x, rest, id))
           case _ => LazyList.empty
       })   
    def smallestRange(nums: List[List[Int]]): Array[Int] =
    go(nums.zipWithIndex.collect{ case (x :: rest, id) => Item(x, rest, id) }.to(TreeSet))
    .minBy{ case Vector(a, b) => b - a }.toArray
}