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]
]
class Solution:
def levelOrderBottom(self, root: TreeNode) -> List[List[int]]:
if not root:
return []
queue = [root]
stack = []
while queue:
next_queue = []
tmp = []
while queue:
n = queue.pop(0)
tmp.append(n.val)
if n.left:
next_queue.append(n.left)
if n.right:
next_queue.append(n.right)
stack.append(tmp[:])
queue = next_queue[:]
return stack[::-1]