NSDictionary+Additions.swift

func highestKey<T, N: Comparable>(of dictionary: Dictionary<T, N>) -> T? {
  var highestKey: T?
  var highestValue: N?

  for key in dictionary.keys {
    let value = dictionary[key]
    if highestValue == nil || value! > highestValue! {
      highestKey = key
      highestValue = value
    }
  }
  return highestKey
}

let dictionary = ["A" : 3, "Z": 6, "D": 7]
let key = highestKey(of: dictionary)
print(key ?? "n/a")