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],

Example: Try out the enumerate function for yourself in this quick exercise. Complete the skip_elements function to return every other element from the list, this time using the enumerate function to check if an element is on an even position or an odd position.

def altElement(a):
    return a[::2]