golf.scala

// given a List[Int], e.g. List(1, 50, 250, 250, 50, 6), find all the pairs of numbers which sum to 500

def pairsOfSumOfN(ns: List[Int], n: Int): List[(Int, Int)] =
  ns.zipWithIndex.map(pair => ns.zipWithIndex.filter(_._2 != pair._2).map(_._1).zipWithIndex.map(second => (pair._1, second._1))).flatMap(a => a.filter(b => n == (b._1 + b._2)))

pairsOfSumOfN(List(1, 50, 250, 250, 50, 6), 500) // returns List((250, 250), (250, 250))! (can we dedupe this list by using indexes? you bet!)

Easy to produce in ~5 minutes with Ammonite. Standing at a whiteboard? Please no.