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]
]
# Definition for a binary tree node.
# class TreeNode(object):
# def __init__(self, x):
# self.val = x
# self.left = None
# self.right = None
from collections import deque
class Solution(object):
def levelOrderBottom(self, root):
"""
:type root: TreeNode
:rtype: List[List[int]]
"""
if not root:
return []
ans = [[root.val]]
queue = deque([root])
while queue:
levelans = []
for _ in xrange(0, len(queue)):
root = queue.popleft()
if root.left:
levelans.append(root.left.val)
queue.append(root.left)
if root.right:
levelans.append(root.right.val)
queue.append(root.right)
if levelans:
ans.append(levelans)
return ans[::-1]
