Binary Tree Upside Down

上下颠倒一个二叉树. 这个题应该是hard难度, 直接看很难发现规律.

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