LeetCode之 107. Binary Tree Level Order Traversal II python

Given a binary tree, return the bottom-up level traversal of its node values. (That is, from the layer where the leaf node is located to the layer where the root node is located, traversing layer by layer from left to right)

For example:
given a binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

Return its bottom-up level 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 root == None:
            return []
        temp=[]
        res=[]
        stack=[]
        n=1
        stack.append(root)
        while stack: #stack is used to traverse all elements of the tree, temp stores the elements of each layer, n judges whether the elements of this layer are processed
            node=stack.pop(0)
            n-=1
            temp+=[node.val]
            if node.left!=None:
                stack.append(node.left)
            if node.right!=None:
                stack.append(node.right)
            if n == 0:
                res.insert(0,temp)
                temp=[]
                n = len (stack)
        return res