题目描述

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.

代码

    public TreeNode invertTree(TreeNode root) {

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

        TreeNode tmp = root.left;
        root.left = root.right;
        root.right = tmp;

        if (root.left != null) {
            invertTree(root.left);
        }

        if (root.right != null) {
            invertTree(root.right);
        }

        return root;
    }