Invert Binary Tree

Invert 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.

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 null;

        TreeNode right = invertTree(root.left);
        TreeNode left = invertTree(root.right);
        root.left = left;
        root.right = right;

        return root;
    }
}