问难以理解为什么在尝试倒置二叉树时必须创建新的临时TreeNode

EN

public class Solution {
        public TreeNode invertTree(TreeNode root) {
        if(root == null) return null;
            TreeNode tmp = root.left;
            root.left = invertTree(root.right);
            root.right = invertTree(tmp);
            return root;
        }
}

public class Solution {
        public TreeNode invertTree(TreeNode root) {
        if(root == null) return null;
            root.left = invertTree(root.right);
            root.right = invertTree(root.left);
            return root;
        }
}