226. 翻转二叉树

题目：
给你一棵二叉树的根节点 root ，翻转这棵二叉树，并返回其根节点。

示例1：

解题思路：
先翻转左子树和右子树，在交换左右子树。

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