1 """
2 首先理解题意，因为要找结点的下个指针
3 不能从根节点来看。要从第二层看来判断
4 找到规律自然用递归写法
5 """
6 class Node:
7 def __init__(self, val: int = 0, left: 'Node' = None, right: 'Node' = None, next: 'Node' = None):
8 self.val = val
9 self.left = left
10 self.right = right
11 self.next = next
12
13 class Solution:
14 def connect(self, root: 'Node') -> 'Node':
15 if not root:
16 return None
17 if root.left:
18 root.left.next = root.right
19 if root.right:
20 root.right.next = root.next.left if root.next else None #！！！从第二层来判断
21 self.connect(root.left)
22 self.connect(root.right)
23 return root
