LeetCode: 107. Binary Tree Level Order Traversal II

题目

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]
]

解题思路

这道题跟LeetCode: 102. Binary Tree Level Order Traversal一样的题意，只是把res反转输出就可以了。反转有两种写法。

# Definition for a binary tree node.
# class TreeNode:
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = Noneclass Solution:def levelOrderBottom(self, root: TreeNode) -> List[List[int]]:res = []if root is None:return reslevel = [root]while level:res.append([node.val for node in level])internode = []for node in level:# internode.extend([node.left, node.right])internode.append(node.left)internode.append(node.right)level = [node for node in internode if node]# return list(reversed(res))return res[::-1]

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LeetCode: 107. Binary Tree Level Order Traversal II

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