Return List of Reverse Levels of Binary Tree

Given a binary tree, return the bottom-up level order traversal of its nodes’ values. (ie, from left to right, level by level from leaf to root).

For example:
Given binary tree [3,9,20,null,null,15,7],

    3
   / \
  9  20
    /  \
   15   7

return its bottom-up level order traversal as:

[
  [15,7],
  [9,20],
  [3]
]

Solution:

Explanation:

We are traversing the Binary tree are storing it in a Queue. Now in each while loop, we will be traversing the entire level, which is handled by the for loop inside the while loop.

Time Complexity: We will traverse all the nodes exactly once, hence the complexity is O(n)

Space Complexity: We will be storing all the nodes in the Queue, hence the space complexity is O(n)