Dictionary+MaxKey.swift

extension Dictionary {
    //only [String:Int]
    func maxKey() -> [String]?{
        guard let values = Array(self.values) as? [Int] else { return nil }
        guard let _ = Array(self.keys) as? [String] else { return nil }
        //valueの最大値を取り出す
        guard let max = values.max() else { return nil }
        
        var maxKeys:[String] = []
        //辞書の中身をひとつずつ見ていく
        for ( key,value ) in self{
            if value as! Int == max{
                print("dic key is \(key) , dic value is \(value)")
                maxKeys.append(key as! String)
            }
        }
        return maxKeys
    }
}