[Swift] 둘만의 암호

import Foundation

func solution(_ s:String, _ skip:String, _ index:Int) -> String {
    let filtered = "abcdefghijklmnopqrstuvwxyz".filter { !skip.contains($0) }.map { $0 }
    let maps = filtered.enumerated().reduce(into: [:]) { dict, v in
        dict[v.element] = filtered[(v.offset+index) % filtered.count]
    }

    return s.map { String(maps[$0]!) }.joined()
}