def __init__(self, name, host='127.0.0.1', port=27017):
        self.name = name
        self.host = host
        self.port = port
def dfs_print(root):
        if root is None:
            return
        dfs_print(root.left)
        print(str(root.key) + ' ', end='')
        dfs_print(root.right)