力扣HOT100 - 226. 翻转二叉树

解题思路：

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;
    }
}