Binary Tree Zigzag Level Order Traversal

Problem

Given a binary tree, return the zigzag level order traversal of its nodes' values. (ie, from left to right, then right to left for the next level and alternate between).
For example:

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Idea

用两个栈实现,依次往s1,s2插入，一个先插左子树再右子树，一个先插右子树再左子树，最后就能得到zigzag的遍历效果

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Solution

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