题目：
题解：
class Solution:
def connect(self, root: 'Node') -> 'Node':
if not root:
return None
start = root
while start:
self.last = None
self.nextStart = None
p = start
while p:
if p.left:
self.handle(p.left)
if p.right:
self.handle(p.right)
p = p.next
start = self.nextStart
return root
def handle(self, p):
if self.last:
self.last.next = p
if not self.nextStart:
self.nextStart = p
self.last = p