17. Letter Combinations of a Phone Number

    let map : [Character : String] = [ "2" : "abc",
                                        "3" : "def",
                                       "4" : "ghi",
                                       "5" : "jkl",
                                       "6" : "mno",
                                       "7" : "pqrs",
                                       "8" : "tuv",
                                       "9" : "wxyz"]

    func letterCombinations(_ digits: String) -> [String] {
        guard digits.count > 0 else { return [String]()}
        let arr = Array(digits)
        var list = [Character]()
        var res = [String]()
        combinations(arr, 0, &list, &res)
        return res
    }
    func combinations(_ arr:[Character], _ index: Int, _ list: inout [Character], _ res: inout [String]){
        if list.count == arr.count{
            res.append(String(list))
            return
        }

        if let currList = map[arr[index]]{
            for char in currList{
                list.append(char)
                combinations(arr, index + 1, &list, &res)
                list.removeLast()
            }
        }
    }