Binary Tree Zigzag Traversal@leetcode

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

    3
   / \
  9  20

return its zigzag level order traversal as:
[
  [3],
  [20,9],
  [15,7]
]
这题有一点儿意思，但和前面那个level traverse也是换汤不换药，顺序改变，于是我们就用stack来做就好了。