Towers of hanoi

gistfile1.swift

// Challenge set here...
// https://blog.svpino.com/2015/06/07/programming-challenge-towers-of-hanoi

import UIKit

// convenience for printing
func moveRing(ring : Int, currentPole: Int, targetPole: Int) {
    println("Move ring \(ring) from pole \(currentPole) to pole \(targetPole)")
}

// recursive function
func towersOfHanoi(rings : [Int], currentPole: Int, targetPole: Int) {
    // only one ring. Just move it to target
    if rings.count == 1 {
        moveRing(rings[0], currentPole, targetPole)
        return
    }
    
    // get all but the bottom ring
    let topRings : [Int] = Array(rings[0..<rings.count - 1])
    
    // set new target to "empty" pole
    let newTarget = [1, 2, 3].filter{
        $0 != currentPole && $0 != targetPole
        }[0]
    
    // move them to the new target
    towersOfHanoi(topRings, currentPole, newTarget)
    
    // move bottom pole to target
    moveRing(rings[rings.count - 1], currentPole, targetPole)
    
    // move other rings to target
    towersOfHanoi(topRings, newTarget, targetPole)
}

let rings = [1, 2, 3, 4]

println("Number of moves = \(Int(pow(2.0, Double(rings.count))) - 1)")

towersOfHanoi(rings, 1, 3)

Output...

Number of moves = 15
Move ring 1 from pole 1 to pole 2
Move ring 2 from pole 1 to pole 3
Move ring 1 from pole 2 to pole 3
Move ring 3 from pole 1 to pole 2
Move ring 1 from pole 3 to pole 1
Move ring 2 from pole 3 to pole 2
Move ring 1 from pole 1 to pole 2
Move ring 4 from pole 1 to pole 3
Move ring 1 from pole 2 to pole 3
Move ring 2 from pole 2 to pole 1
Move ring 1 from pole 3 to pole 1
Move ring 3 from pole 2 to pole 3
Move ring 1 from pole 1 to pole 2
Move ring 2 from pole 1 to pole 3
Move ring 1 from pole 2 to pole 3

Awesome!

Updated to use a better (shorter) "new target" method.

Further improved