103. Binary Tree Zigzag Level Order Traversal

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,null,null,15,7],
     3
    / \
   9  20
      / \
    15  7

return its zigzag level order traversal as:
[ [3], [20,9], [15,7] ]