Good morning! Here's our prompt for today.

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:

1     4
2   /   \
3  2     7
4 / \   / \
51   3 6   9

Performing an inversion would result in:

1Output:
2
3     4
4   /   \
5  7     2
6 / \   / \
79   6 3   1

The definition of a tree node is as follows:

1function Node(val) {
2  this.val = val;
3  this.left = null;
4  this.right = null;
5}

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)

Try to solve this here or in Interactive Mode.

How do I practice this challenge?

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  assertIsFunction(invertTree, '`invertTree` should be a function');

var assert = require('assert');

​

function invertTree(head) {

  return;

}

​

function Node(val) {

  this.val = val;

  this.left = null;

  this.right = null;

}

​

// Regular binary trees

var tree1 = new Node(4);

tree1.left = new Node(1);

tree1.right = new Node(3);

​

var tree2 = new Node(5);

tree2.left = new Node(10);

tree2.left.left = new Node(17);

tree2.left.right = new Node(3);

tree2.right = new Node(8);

​

// Binary search trees

var tree3 = new Node(6);

tree3.left = new Node(3);

​

var tree4 = new Node(5);

tree4.left = new Node(3);

Here's a video of us explaining the solution.

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We'll now take you through what you need to know.

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