1006. insert for single link list with head node

带虚拟头结点的单链表结点结构如下：

struct ListNode
{
int data;
ListNode *next;
};
链表类接口如下：
class List
{
public:
List()
{
head = new ListNode;
head->next = NULL;
}

~List()
{
ListNode* curNode;
while( head->next )
{
curNode = head->next;
head->next = curNode->next;
delete curNode;
}
delete head;
}
//在链表第pos(pos>0)个结点之前插入新结点，新结点的值为toadd
//链表实际结点从1开始计数。
//调用时需保证pos小等于链表实际结点数
void List::insert(int toadd, int pos);

// Data field
ListNode *head; //head指向虚拟头结点，head-next指向第一个实际结点
};

请实现如下函数：
void List::insert(int toadd, int pos)

只提交insert函数实现，不需要提交类定义及main函数。
Problem Source: LXM

我的代码

// Problem#: 19147// Submission#: 4809302// The source code is licensed under Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License// URI: http://creativecommons.org/licenses/by-nc-sa/3.0/// All Copyright reserved by Informatic Lab of Sun Yat-sen Universityvoid List::insert(int toadd, int pos) {    ListNode *pre = head;    if (pos <= 0) return;    while (--pos && pre->next != NULL) {        pre = pre->next;    }    ListNode * add = new ListNode;    add->data = toadd;    if (pre->next != NULL) {        add->next = pre->next;    } else {        add->next = NULL;    }    pre->next = add;}