Here is the interview question prompt, presented for reference.

Can you invert a binary tree over its vertical axis? This is a famous problem made popular by this tweet:

Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so f**k off.— Max Howell (@mxcl) June 10, 2015

Given a binary tree like this:

     4
   /   \
  2     7
 / \   / \
1   3 6   9

Performing an inversion would result in:

Output:

     4
   /   \
  7     2
 / \   / \
9   6 3   1

The definition of a tree node is as follows:

function Node(val) {
  this.val = val;
  this.left = null;
  this.right = null;
}

Constraints

• Number of nodes in the tree <= 1000
• The nodes will always contain integer values between -1000000000 and 1000000000
• Expected time complexity : O(n)
• Expected space complexity : O(logn)

You can see the full challenge with visuals at this link.