剑指 Offer 32 - II. 从上到下打印二叉树 II
说明
从上到下按层打印二叉树，同一层的节点按从左到右的顺序打印，每一层打印到一行。
示例
给定二叉树: [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
返回其层次遍历结果：
[
[3],
[9,20],
[15,7]
]
题解思路
在从上到下打印二叉树的前提下按层输出
代码实现
# 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 not root:
return []
queue = collections.deque()
ans = []
queue.append(root)
while queue:
n = len(queue)
tmp = []
for i in range(n):
node = queue.popleft()
tmp.append(node.val)
left, right = node.left, node.right
if left:
queue.append(left)
if right:
queue.append(right)
ans.append(tmp)
return ans
