ListNode* LastNNode(ListNode *root, int n)
{
    if (root == NULL || n <=0)
        return NULL;
    ListNode *cur = root;
    ListNode *pre = root;
    int cnt = 0;
    while (pre != NULL)
    {
        cnt++;
        if (cnt == n)
            break;
        pre = pre->next;
    }
    if (cnt < n)
    {
        return NULL;
    }
    else
    {
        cur = root;
        while (pre->next != NULL)
        {
            pre = pre->next;
            cur = cur->next;
        }
        return cur;
    }
}