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).
Given binary tree
3 / \ 9 20 / \ 15 7
return its zigzag level order traversal as:
[ , [20,9], [15,7] ]
Similar to Binary Tree Level Order Traversal except that we need to reverse the even rows.