leetcode 面试题32-III 从上到下打印二叉树

思路：同样是用队列层次遍历二叉树，遍历完完整的一层，再添加到返回结果中，奇数行正序返回，偶数行逆序返回

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution:
def levelOrder(self, root: TreeNode) -> List[List[int]]:
if root is None:
return []
import queue
q = queue.Queue()
q.put(root)
res = []
while(not q.empty()):
tmp = []
for i in range(q.qsize()):
cur = q.get()
tmp.append(cur.val)
if cur.left:
q.put(cur.left)
if cur.right:
q.put(cur.right)
if len(res) % 2 == 0:#奇数行正序返回，偶数行逆序返回
res.append(tmp)
else:
res.append(tmp[::-1])
return res