root.right = construct_tree(preorder[index + 1:], right)

index = inorder.index(preorder[0])
root = TreeNode(preorder[0])
root.left = construct_tree(preorder[1:index + 1], inorder[0:index])
root.right = construct_tree(preorder[index + 1:], inorder[index + 1:])
return root

import time

class SingleTon(object):
    _instance = {}
    def __new__(cls, *args, **kwargs):
        print(threading.current_thread())
        if cls not in cls._instance:
            print('Singleton __new__ is true')
            time.sleep(15)
            cls._instance[cls] = super(SingleTon, cls).__new__(cls, *args, **kwargs)

        return cls._instance[cls]

class Test(SingleTon):
    def __init__(self):
        print('test __init__')
        self.val = 2

def running():
    t1 = Test()
    print(id(t1))

import threading

th1 = threading.Thread(target=running, name='th1')
th2 = threading.Thread(target=running, name='th2')
th1.start()
th2.start()
##### 输出 #####
<Thread(th1, started 2736)>
Singleton __new__ is true
<Thread(th2, started 3728)>
Singleton __new__ is true
test __init__
2210173698000
test __init__
2210173726848