Leetcode: Binary Tree Zigzag Level Order Traversal @Python

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

class Solution:
    # @param root, a tree node
    # @return a list of lists of integers
    def zigzagLevelOrder(self, root):
        crt=[root]
        rst=[]
        if not crt:
            return []
        flag=1
        while crt!=[None]*len(crt):
            if flag==1:
                rst.append([i.val for i in crt])
            else:
                temp=[i.val for i in crt]
                temp.reverse()
                rst.append(temp)
            nextlvl=[]
            for i in crt:
                if i.left:
                    nextlvl.append(i.left)
                if i.right:
                    nextlvl.append(i.right)
            crt=nextlvl
            flag*=-1
        return rst