Leetcode 102 Binary Tree Level Order Traversal

Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level).

For example:
Given binary tree {3,9,20,#,#,15,7},

    3
   / \
  9  20
    /  \
   15   7

 

return its level order traversal as:

[
  [3],
  [9,20],
  [15,7]
] 

class Solution(object):
    def levelOrder(self, root):
        if not root:
            return []
        stack, ans = [root], []
        while stack:
            temp, stack_new = [], []
            for x in stack:
                temp.append(x.val)
                if x.left:
                    stack_new.append(x.left)
                if x.right:
                    stack_new.append(x.right)
            ans.append(temp)
            stack = stack_new
        return ans