프로그래머스 - 숫자 짝꿍

import Foundation

func solution(_ X:String, _ Y:String) -> String {
    let s: Set<Character> = Set(X)
    var arr: [String] = []
    
    for i in s {
        let xcount = X.filter { $0 == i }.count
        let ycount = Y.filter { $0 == i }.count
        arr.append(String(repeating: i, count: xcount < ycount ? xcount : ycount))
    }
    let result = arr.sorted().reversed().joined()
    return result.isEmpty ? "-1" : (result.first == "0" ? "0" : result)
}