Sequence extension for group by feature

Sequence+GroupBy.swift

extension Sequence {
    func grouped<T>(by criteria: (Element) -> T) -> [T: [Element]] {
        var dict = [T: [Element]]()
        for element in self {
            let key = criteria(element)
            if dict.keys.contains(key) == false {
                dict[key] = [Element]()
            }
            dict[key]?.append(element)
        }
        return dict
    }
}