SICP exercise 2.40.

UniquePairs.scala

import scala.collection.mutable.ListBuffer

/**
  * Created by User on 11/9/2016.
  */
object UniquePairs {

  def uniquePairs(n: Int): List[List[Int]] = {
    (1 to n).flatMap(i => (1 until i).map(j => List(i, j))).toList
  }

  def main(args: Array[String]): Unit = {
    println(uniquePairs(5).mkString("\n"))
  }
}