Facebook Interview Question: Telephone Passphrase Equivalents

TelephonePassphraseEquivalents

// A telephone keypad has letters associated with each number  (e.g. 2 = abc, 3 = def).
// Given a passphrase of "fb1" (e.g. one that you might use to log into a bank account),
// come up with an algorithm that would assemble an array that contains all the
// different possible letter combinations that, when typed into a telephone dial pad,
// would be equivalent to the original passphrase.  That is, "fb1" equals "321" numerically;
// matching equivalent combinations include: "da1", "db1", "dc1", "ea1", "eb1", "ec1", "fa1" and "fc1".

// This can be dropped into an iOS Playground in Xcode

import UIKit

let inputPassphrase = "fb1"

func add(these digits : [Character], toThis currentOutputArray : [String]) -> [String]
{
    var newOutputArray = [String]()
    
    for eachDigit in digits
    {
        if currentOutputArray.count > 0
        {
            for eachEntry in currentOutputArray
            {
                newOutputArray.append(eachEntry + String(eachDigit))
            }
        } else {
            newOutputArray.append(String(eachDigit))
        }
    }
    return newOutputArray
}

// as long as you know the input is UTF8!
let inputPassphraseArray = Array(inputPassphrase)
var numericEquivalentArray = [Int]()

var outputArray = [String]()

// convert the passphrase into the numeric equivalent
for eachCharacter in inputPassphraseArray
{
    switch eachCharacter
    {
    case Character("a"), Character("b"), Character("c") :
        numericEquivalentArray.append(2)
    case Character("d"), Character("e"), Character("f") :
        numericEquivalentArray.append(3)
    case Character("g"), Character("h"), Character("i") :
        numericEquivalentArray.append(4)
    case Character("j"), Character("k"), Character("l") :
        numericEquivalentArray.append(5)
    case Character("m"), Character("n"), Character("o") :
        numericEquivalentArray.append(6)
    case Character("p"), Character("q"), Character("r"), Character("s") :
        numericEquivalentArray.append(7)
    case Character("t"), Character("u"), Character("o") :
        numericEquivalentArray.append(8)
    case Character("w"), Character("x"), Character("y"), Character("z") :
        numericEquivalentArray.append(9)
    case Character("0") :
        numericEquivalentArray.append(0)
    case Character("1") :
        numericEquivalentArray.append(1)
    default :
        print("no equivalent here!")
    }
}

// now for each digit in the numeric equivalent array, append each letter to the current output array
for eachDigit in numericEquivalentArray
{
    switch eachDigit
    {
    case 0 :
        outputArray = add(these: [Character("0")], toThis: outputArray)
    case 1 :
        outputArray = add(these: [Character("1")], toThis: outputArray)
    case 2 :
        outputArray = add(these: [Character("a"), Character("b"), Character("c")], toThis: outputArray)
    case 3 :
        outputArray = add(these: [Character("d"), Character("e"), Character("f")], toThis: outputArray)
    case 4 :
        outputArray = add(these: [Character("g"), Character("h"), Character("i")], toThis: outputArray)
    case 5 :
        outputArray = add(these: [Character("j"), Character("k"), Character("l")], toThis: outputArray)
    case 6 :
        outputArray = add(these: [Character("m"), Character("n"), Character("o")], toThis: outputArray)
    case 7 :
        outputArray = add(these: [Character("p"), Character("q"), Character("r"), Character("s")], toThis: outputArray)
    case 8 :
        outputArray = add(these: [Character("t"), Character("u"), Character("v")], toThis: outputArray)
    case 9 :
        outputArray = add(these: [Character("w"), Character("x"), Character("y"), Character("z")], toThis: outputArray)
    default :
        print("unexpected digit")
    }
}

print("outputArray is \(outputArray)")