Resolution of a Facebook iOS Code Challenge.

equivalentStrings.swift

import Foundation

/*
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".
*/

func getEquivalentStrings(for string: String) -> [String] {
    
    let keypadDictionary = [1: ["1"],
                            2: ["a","b","c"],
                            3: ["d","e","f"],
                            4: ["g","h","i"],
                            5: ["j","k","l"],
                            6: ["m","n","o"],
                            7: ["p","q","r","s"],
                                8: ["t","u","v"],
                                9: ["w","x","y","z"],
                                0: ["0"]]
    
    let characterStrings = string.map { "\($0)" } //[String]
    
    let relatedLettersArray = characterStrings.map { characterString in Array(keypadDictionary.values).filter { $0.contains { $0 == characterString }}.first! } //[[String]]
    
    
    func allCombinations(for array: [[String]]) -> [String] {
        
        guard let firstArray = array.first else {
            
            return [""]
        }
        
        return firstArray.flatMap { string in allCombinations(for: Array(array.dropFirst())).map { string + $0 } }
    }
    
    return allCombinations(for: relatedLettersArray).filter { $0 != string } //[String]
}

let pass = "fb1"

let equivalentPassStrings = getEquivalentStrings(for: pass)
print(equivalentPassStrings)