Leetcode: Invert Binary Tree

nvert a binary tree.

     4
   /   \
  2     7
 / \   / \
1   3 6   9
to
     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 fuck off.

Of course use Recursion, on each level, have the root.left switch to right and root.right switch to left

public class Solution {
    public TreeNode invertTree(TreeNode root) {
        if (root == null) return null;
        TreeNode left = root.left;
        root.left = invertTree(root.right);
        root.right = invertTree(left);
        return root;
    }
}