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]
]

# Definition for a binary tree node.
# class TreeNode(object):
#     def __init__(self, x):
#         self.val = x
#         self.left = None
#         self.right = None

class Solution(object):
    def levelOrderBottom(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        if not root:
            return []
        ans=[]
        levelans=[]
        que=[root]
        
        while que:
            n=len(que)
            i=0
            while i<n:
                node=que.pop(0)
                levelans.append(node.val)
                if node.left:
                    que.append(node.left)
                if node.right:
                    que.append(node.right)
                i+=1
            ans.append(levelans)
            levelans=[]
        return ans[::-1]