Description
Given the root of a binary tree, return the bottom-up level order traversal of its nodes' values. (i.e., from left to right, level by level from leaf to root).
Example 1:
Input: root = [3,9,20,null,null,15,7] Output: [[15,7],[9,20],[3]]
Example 2:
Input: root = [1] Output: [[1]]
Example 3:
Input: root = [] Output: []
Constraints:
- The number of nodes in the tree is in the range
[0, 2000]. -1000 <= Node.val <= 1000
Solution
Python3
# Definition for a binary tree node.
# class TreeNode:
# def __init__(self, val=0, left=None, right=None):
# self.val = val
# self.left = left
# self.right = right
class Solution:
def levelOrderBottom(self, root):
q, res = collections.deque([(root, 0)]), []
while q:
node, lvl = q.popleft()
if node:
if len(res) < lvl + 1:
res.insert(0, [])
res[-(lvl+1)].append(node.val)
for i in (node.left,node.right):
if i:
q.append([i, lvl+1])
return res