226. Invert Binary Tree

Invert a binary tree.

Example:

Input:

     4
   /   
  2     7
 /    / 
1   3 6   9

Output:

     4
   /   
  7     2
 /    / 
9   6 3   1

Trivia:
This problem was inspired by this original tweet by Max Howell:

Google: 90% of our engineers use the software you wrote (Homebrew), but you can’t invert a binary tree on a whiteboard so f*** off.

Runtime: 92 ms, faster than 96.94% of C# online submissions for Invert Binary Tree.

Memory Usage: 22.1 MB, less than 28.89% of C# online submissions for Invert Binary Tree.

  public TreeNode InvertTree(TreeNode root)
        {
            if (root == null)
            {
                return null;
            }

            var left = root.left;
            var right = root.right;
            if (left == null && right == null)
            {
                return root;
            }

            if (left != null)
            {
                root.right = InvertTree(left);
            }
            else
            {
                root.right = null;
            }

            if (right != null)
            {
                root.left = InvertTree(right);
            }
            else
            {
                root.left = null;
            }

            return root;
        }