17. Letter Combinations of a Phone Number

17. Letter Combinations of a Phone Number.swift

class Solution {
    let phoneDict: [Int : String] = [
        2 : "abc",
        3 : "def",
        4 : "ghi",
        5 : "jkl",
        6 : "mno",
        7 : "pqrs",
        8 : "tuv",
        9 : "wxyz"
    ]
    func letterCombinations(_ digits: String) -> [String] {
        
        var combinations = [String]()
        guard digits.isEmpty != true else {
            return combinations
        }
        for digit in digits {
            guard let number = Int(String(digit)), let letters = phoneDict[number] else {
                continue
            }
            var newCombinations = [String]()
            for letter in letters {
                if combinations.count == 0 {
                    newCombinations.append(String(letter))
                } else {
                    newCombinations = newCombinations + combinations.map({$0 + String(letter)})
                }
            }
            combinations = newCombinations
        }
        return combinations
    }
}

let sol = Solution()
let val = "23"
print("\n\(sol.letterCombinations(val))")