226. 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

来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/invert-binary-tree

public class InvertBinaryTree {
    static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int x) {
            val = x;
        }
    }
    //方式一：到叶子节点就返回
    public TreeNode invertTree1(TreeNode root) {
        if(root == null || (root.left == null && root.right == null)) {
            return root;
        }
        TreeNode leftNode = invertTree1(root.left);  //找到左结点
        TreeNode rightNode = invertTree1(root.right);  //找到右结点
        root.left = rightNode;  //交换节点
        root.right = leftNode;
        return root;
    }
    //方式二：直到null才返回
    public TreeNode invertTree2(TreeNode root) {
        if(root == null) {
            return null;
        }
        TreeNode leftNode = root.left;
        root.left = invertTree2(root.right);
        root.right = invertTree2(leftNode);
        return root;
    }
}