def __init__(self, name, host='127.0.0.1', port=27017):
        self.name = name
        self.host = host
        self.port = port
def kth(root, k):
    if k < size(root.left):
        return kth(root.left, k)
    elif k > size(root.left):
        return kth(root.right, k - size(root.left) - 1)
    return root.key