I want to replace duplicate characters in string with one char and others which are not duplicate with another char. So output will have string consisting with only 2 chars.

This is what I came up with so far...

import Foundation

func encode(myString:String) -> String{
    let mySet = Set(myString)
    if mySet.count == myString.count{
        return String(Array(repeating: ".", count: myString.count))
    }
    else {
        var outputString = ""
        for (position,letter) in myString.enumerated() {
            if position == myString.count - 1 {
                break // last char is guaranteed to be unique
            }
                outputString = myString.replacingOccurrences(of: String(letter), with: "X")
            }
        for letter in outputString{
            if letter != "X"{
                outputString = outputString.replacingOccurrences(of: String(letter), with: ".")
            }
        }

        return outputString
    }
}

encode(myString: "hello") // will print ..XX.
encode(myString:"helloo"//will print ....XX which is wrong