Perfect Binary Tree Specific Level Order Traversal

Given a Perfect Binary Tree like below: 

👁 image(4)

Print the level order of nodes in following specific manner:  

 1 2 3 4 7 5 6 8 15 9 14 10 13 11 12 16 31 17 30 18 29 19 28 20 27 21 26 22 25 23 24

i.e. print nodes in level order but nodes should be from left and 
right side alternatively. Here 1st and 2nd levels are trivial. 
While 3rd level: 4(left), 7(right), 5(left), 6(right) are printed. 
While 4th level: 8(left), 15(right), 9(left), 14(right), .. are printed. 
While 5th level: 16(left), 31(right), 17(left), 30(right), .. are printed.

We strongly recommend to minimize your browser and try this yourself first.
In standard Level Order Traversal, we enqueue root into a queue 1^st, then we dequeue ONE node from queue, process (print) it, enqueue its children into queue. We keep doing this until queue is empty.

Approach 1:  We can do standard level order traversal here too but instead of printing nodes directly, we have to store nodes in current level in a temporary array or list 1^st and then take nodes from alternate ends (left and right) and print nodes. Keep repeating this for all levels. 
This approach takes more memory than standard traversal.

Specific Level Order traversal of binary tree is 
1 2 3 4 7 5 6 8 15 9 14 10 13 11 12 16 31 17 30 18 29 19 28 20 27 21 26 22 25 23 24 

Time Complexity: O(N) where N is the number of nodes in given binary tree.
Auxiliary Space: O(N) due to queue and vector data structure.

Approach 2: The standard level order traversal idea will slightly change here. Instead of processing ONE node at a time, we will process TWO nodes at a time. And while pushing children into queue, the enqueue order will be: 1^st node's left child, 2^nd node's right child, 1^st node's right child and 2^nd node's left child.

Implementation:

Specific Level Order traversal of binary tree is 
1 2 3 4 7 5 6 8 15 9 14 10 13 11 12 16 31 17 30 18 29 19 28 20 27 21 26 22 25 23 24

Time Complexity: O(n) where n is the total number of nodes in the tree. 
Auxiliary Space : O(n)

Followup Questions: 

1. The above code prints specific level order from TOP to BOTTOM. How will you do specific level order traversal from BOTTOM to TOP (Amazon Interview | Set 120 - Round 1 Last Problem)
2. What if tree is not perfect, but complete.
3. What if tree is neither perfect, nor complete. It can be any general binary tree.