leetcode107. 二叉树的层次遍历 II

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https://leetcode-cn.com/problems/binary-tree-level-order-traversal-ii
自底向上层次遍历：

# 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):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        res = []
        stack = [root]
        while stack:
            res0 = []
            for i in range(len(stack)):
                node = stack.pop(0)
                if node:
                    stack.extend([node.left, node.right])
                    res0.append(node.val)
            if res0:
                res.append(res0)
        res.reverse()
        return res