翻转二叉树

翻转一棵二叉树。

示例：

输入：

 4

/
2 7
/ \ /
1 3 6 9

输出：

 4

/
7 2
/ \ /
9 6 3 1

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