二叉搜索树与双向链表：Leetcode上完全可以通过，但这里为何不行？

public class Solution {
    TreeNode pre, head;
    public TreeNode Convert(TreeNode root) {
            if(root == null) return null;
            dfs(root);
            head.left = pre;
            pre.right = head;
            return head;
        }
        void dfs(TreeNode cur) {
            if(cur == null) return;
            dfs(cur.left);
            if(pre != null) pre.right = cur;
            else head = cur;
            cur.left = pre;
            pre = cur;
            dfs(cur.right);
        }
}

我想知道我哪里错了🤣🤣🤣🤣🤣麻烦大神指点一下；