AddByThree.swift

/*Algorithm spec:

Given a long list of numbers: 
let numbers = 1...1000000

Write a function that transforms the list of numbers to strings with the following mapping and put it into a variable called results.

1->A, 2->B, 3->C ... 10->A0, 11->AA, 12->AB etc.

With the results from the last part, write a function that adds the following elements.
addedByThree = results[3] + results[33] + results[333] + ...
*/

func mapper<C: Collection>(list: C) -> [String] {
    var results: [String] = []
    for number in list {
        var mapped_element = ""
        let array = (String(describing: number)).characters.flatMap{Int(String($0))}
        for elem in array {
            switch elem {
                case 1: 
                    mapped_element += "A"
                case 2:
                    mapped_element += "B"
                case 3: 
                    mapped_element += "C"
                case 4:
                    mapped_element += "D"
                case 5: 
                    mapped_element += "E"
                case 6:
                    mapped_element += "F"
                case 7: 
                    mapped_element += "G"
                case 8:
                    mapped_element += "H"
                case 9: 
                    mapped_element += "I"
                case 0:
                    mapped_element += "0"
                default:
                    break
            }
        }
        results += [mapped_element]
    }
    return results
}


func addedByThree(numbers: [String]) -> String {
    var threeElements: [String] = []
    var numberOfDigits = 1
    var currentString = ""
    while numberOfDigits < String(describing: numbers.count).characters.count {
        currentString += "3"
        threeElements += [currentString]
        numberOfDigits += 1
    }
    var finalString = ""
    for elem in threeElements {
        finalString += numbers[Int(elem)!]
    }
    return finalString
}

//print(addedByThree(numbers: mapper(list: 1...100)))