leecode刷题 翻转二叉树

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：

输入：root = [4,2,7,1,3,6,9]
输出：[4,7,2,9,6,3,1]
示例 2：

输入：root = [2,1,3]
输出：[2,3,1]
示例 3：

输入：root = []
输出：[]

提示：

树中节点数目范围在 [0, 100] 内
-100 <= Node.val <= 100

/**
 * 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 root;
        TreeNode left = root.left, right = root.right;
        root.left = invertTree(right);
        root.right = invertTree(left);

        return root;
    }
}