func removeDupLetters(from text: String) -> String {

    //Turn each letter in (Int,Char), assigning a unique number to every letter
    let text = text.enumerated()
    //Gouping text by letter into a dictionary; loses order
    let textDict = Dictionary(grouping: text, by: {$0.element})
    //Returning the first value(Int,Char) of every key(letter), and sorting incremently by the enumerated value
    let singles = textDict.compactMap({$0.value.first}).sorted(by: {$0.offset < $1.offset})

    textDict.forEach({print($0)})
    print("")
    singles.forEach({print($0)})
    print("------")

    return String(singles.compactMap({$0.element}))
}

removeDupLetters(from: "hello")
removeDupLetters(from: "wombat")
removeDupLetters(from: "Mississippi")