107.二叉树的层次遍历-ii.py

#
# @lc app=leetcode.cn id=107 lang=python3
#
# [107] 二叉树的层次遍历 II
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# https://leetcode-cn.com/problems/binary-tree-level-order-traversal-ii/description/
#
# algorithms
# Easy (63.30%)
# Likes:    188
# Dislikes: 0
# Total Accepted:    41.4K
# Total Submissions: 64.4K
# Testcase Example:  '[3,9,20,null,null,15,7]'
#
# 给定一个二叉树，返回其节点值自底向上的层次遍历。 （即按从叶子节点所在层到根节点所在的层，逐层从左向右遍历）
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# 例如：
# 给定二叉树 [3,9,20,null,null,15,7],
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# ⁠   3
# ⁠  / \
# ⁠ 9  20
# ⁠   /  \
# ⁠  15   7
#
#
# 返回其自底向上的层次遍历为：
#
# [
# ⁠ [15,7],
# ⁠ [9,20],
# ⁠ [3]
# ]
#
#
#
from typing import List


# @lc code=start
# Definition for a binary tree node.
class TreeNode:

    def __init__(self, x):
        self.val = x
        self.left = None
        self.right = None


class Solution:

    def levelOrderBottom(self, root: TreeNode) -> List[List[int]]:
        """
        1. 层序遍历，然后逆转 或 deque.appendleft
        """
        from collections import deque
        res = deque()

        lvl = [root]
        while any(lvl):
            res.appendleft([node.val for node in lvl])
            lvl = [child for node in lvl for child in [node.left, node.right] if child]
        return list(res)


# @lc code=end