103. Binary Tree Zigzag Level Order Traversal

def zigzagLevelOrder(self, root):
        """
        :type root: TreeNode
        :rtype: List[List[int]]
        """
        if not root: return []
        res, queue, flag = [], [root], 1
        while queue:
            temp = []
            for i in xrange(len(queue)):
                node = queue.pop(0)
                temp.append(node.val)
                if node.left: queue.append(node.left)
                if node.right: queue.append(node.right)
            res += [temp[::flag]]
            flag *= -1
        return res