我的方法，使用顏色層次遍歷法，依次填next：
"""
# Definition for a Node.
class Node:
def __init__(self, val: int = 0, left: 'Node' = None, right: 'Node' = None, next: 'Node' = None):
self.val = val
self.left = left
self.right = right
self.next = next
"""
class Solution:
def connect(self, root: 'Node') -> 'Node':
white = 1
gray = 0
level = 0
stack = [(white, root, level),]
res = []
if root is None:return
while stack:
color, root, level = stack.pop()
if root is None: continue
if color:
stack.append((white, root.right, level+1))
stack.append((white, root.left, level+1))
stack.append((gray, root, level))
else:
if res.__len__() < level + 1:
res.append([root])
elif res[level].__len__() == 2 ** (level) - 1:
res[level][0].next = root
root.next = None
else:
res[level][0].next = root
res[level][0] = root
return res[0][0]
遞歸法：
"""
# Definition for a Node.
class Node:
def __init__(self, val: int = 0, left: 'Node' = None, right: 'Node' = None, next: 'Node' = None):
self.val = val
self.left = left
self.right = right
self.next = next
"""
class Solution:
def connect(self, root: 'Node') -> 'Node':
if root is None: #針對輸入爲[]這種特殊情況
return root
if root.left is None and root.next is None: #針對輸入爲[1]這種情況
root.next = None
return root
if root.left is None: #不會讓程序訪問到最後一層，在倒數第二層就停了
return root
if root.next is None:
root.left.next = root.right
root.right.next = None
else:
root.left.next = root.right
root.right.next = root.next.left
self.connect(root.left)
self.connect(root.right)
return root