107. Binary Tree Level Order Traversal II

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]
]

Code：

# 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):
        stack = [(root, 0)]
        res = []
        while stack:
            node, level = stack.pop()
            if node:
                if len(res) < level+1:
                    res.insert(0, [])
                res[-(level+1)].append(node.val)
                stack.append((node.right, level+1))
                stack.append((node.left, level+1))
        return res