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