ArrayToOccurrencesDictionary.swift

extension Array where Element: Hashable {
    var occurrencesCount :[Element : Int] {
        var map = [Element : Int]()
        forEach { element in
            if let val = map[element]{
                map[element] = val+1
            } else {
                map[element] = 1
            }
        }
        return map
    }
}