Apparently, the author of Homebrew couldn’t solve this problem while interviewing for a job at Google, I gave it a shot with the comfort a living room and no time pressure.

Problem:
https://leetcode.com/problems/invert-binary-tree/

Solution:

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public TreeNode invertTree(TreeNode root) {
        if (root == null)
	        return root;
        TreeNode temp = root.left;
        root.left = root.right;
        root.right = temp;
        invertTree(root.right);
        invertTree(root.left);
        return root;
    }
}