- Binary Tree Level Order Traversal II Easy
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Share Given a binary tree, return the bottom-up level order traversal of its nodes' values. (ie, from left to right, level by level from leaf to root).
For example:
Given binary tree [3,9,20,null,null,15,7],
3
/
9 20
/
15 7
return its bottom-up level order traversal as:
[
[15,7],
[9,20],
[3]
]
思路：一层一个数组用来存放元素，明显用广度优先搜索。python配合deque用
代码:python3
from collections import deque
class Solution:
def levelOrderBottom(self, root: TreeNode) -> List[List[int]]:
q=collections.deque([(root,0)])
arr=[]
while q:
node,level=q.popleft()
if node:
if len(arr) < level+1:
arr.append([])
arr[level].append(node.val)
q.append((node.left,level+1))
q.append((node.right,level+1))
return arr[::-1]
