Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).
For example:
Given binary tree [3,9,20,null,null,15,7],
3
/ \
9 20
/ \
15 7
return its level order traversal as:
[
[3],
[9,20],
[15,7]
]
注意要找到當前層的所有節點繼續向下遍歷
# 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: # NOTE: 注意root可能爲空
return []
res = []
cur_nodes = [root]
next_nodes = []
res.append([i.val for i in cur_nodes])
while cur_nodes or next_nodes:
for node in cur_nodes:
if node.left:
next_nodes.append(node.left)
if node.right:
next_nodes.append(node.right)
if next_nodes:
res.append(
[i.val for i in next_nodes]
)
cur_nodes = next_nodes
next_nodes = []
return res