Collatz Conjecture - https://en.wikipedia.org/wiki/Collatz_conjecture

collatz.swift

//: Playground - noun: a place where people can play

import UIKit

var hits = [Int: Bool]()

func collatz(_ n: Int) {
    if n == 1 { return }
    var new: Int!
    if (n % 2) == 0 {
        new = n / 2
    }
    else {
        new = 3*n + 1
    }
    if hits[n] == nil {
        hits[n] = true
        print("\(n)")
    }
    collatz(new)
}

collatz(2357693)