OrderedDict.swift

import Foundation

struct OrderedDict<K: Hashable, V> {
    var _dict = [K : V]()
    var _ordered = [K]()
    
    var keys: [K] {
        _ordered
    }
    
    subscript(index: K) -> V? {
        get {
            _dict[index]
        }
        set(newValue) {
            if let _ = newValue {
                if _ordered.firstIndex(of: index) == nil {
                    _ordered.append(index)
                }
            }
            else {
                if let firstIndex = _ordered.firstIndex(of: index) {
                    _ordered.remove(at: firstIndex)
                }
            }
            _dict[index] = newValue
        }
    }
}

var dict = OrderedDict<String, Int>()

dict["one"] = 1
dict["two"] = 2
dict["three"] = 3
dict["four"] = 4
dict["five"] = 5
dict["three"] = nil
dict["three"] = 3

print(dict.keys)