HiBooks.swift

//A standard phone number pad has the following layout.
//1 2 3
//4 5 6
//7 8 9
//  0
//Using this layout and a starting digit we can generate numbers as follows: The first digit is the
//starting digit and each subsequent digit is directly left, right, above or below the previous digit on
//the number pad. For example if the starting digit is 1, 1256 is a valid number, but 1289 is not
//valid because 8 is not directly next to 2. Write a function that takes a starting digit d and an
//integer n as input and returns a list of all unique numbers of length n generated in this way.
//Test cases:
//f(5, 1) = [5]
//f(1, 3) = [121, 123, 125, 141, 145, 147]

import Foundation

func printValidDigits(_ startDigit:Int, _ numberOfRemainingMoves:Int,_ moves:[Int]) {
    
    if numberOfRemainingMoves <= 0 {
        fatalError("invalid number of moves")
    }
    if numberOfRemainingMoves == 1 {
        print(moves + [startDigit])
        return
    }
    
    var options:[Int] = []
    switch startDigit {
    case 0:
        options = [8]
    case 9:
        options = [6,8]
    case 8:
        options = [7,5,9,0]
    case 7:
        options = [4,8]
    case 6:
        options = [3,5,9]
    case 5:
        options = [4,2,6,8]
    case 4:
        options = [1,5,7]
    case 3:
        options = [2,6]
    case 2:
        options = [1,5,3]
    case 1:
        options = [2,4]
    default:
        fatalError("invalid number")
    }
    
    for option in options {
        printValidDigits(option,numberOfRemainingMoves-1, moves + [startDigit])
    }
    
}

func printValidSequences(_ startDigit:Int,_ numberOfRemainingMoves:Int){
    printValidDigits(startDigit,numberOfRemainingMoves,[])
}


printValidSequences(1,3)
printValidSequences(5,1)