Remove Linked List Elements

Description：

Remove all elements from a linked list of integers that have value val.

Example
Given: 1 --> 2 --> 6 --> 3 --> 4 --> 5 --> 6, val = 6
Return: 1 --> 2 --> 3 --> 4 --> 5

Code1：

  ListNode* removeElements(ListNode* head, int val) {
        if (head==NULL)
        {
            return NULL;
        }
        
        //在链表两端分别插入头结点和尾结点
        ListNode* headNode = new ListNode(0);
        headNode->next = head;
        
        ListNode* q = headNode;
        while (q->next)
        {
            q = q->next;
        }
        ListNode* tailNode = new ListNode(0);
        q->next = tailNode;
        
        q = head;
        ListNode* preNode = headNode;//指向被删结点的上一个结点
        while (q!=tailNode)
        {
            if (q->val == val)
            {
                preNode->next = q->next;
                delete q;
                q = preNode->next;
            }
            else
            {
                q=q->next;
                preNode=preNode->next;
            }
        }
        
        if (headNode == tailNode)
            return NULL;
        else
        {
            preNode->next = NULL;
            head = headNode->next;
            delete headNode;
            delete tailNode;
            return head;
        }
    }

Code2:

  ListNode* removeElements(ListNode* head, int val) {
        if (head==NULL)
        {
            return NULL;
        }

        ListNode* q = head;
        ListNode* preNode = NULL;//指向被删结点的上一个结点
        while (q!=NULL)
        {
            if (q->val == val)
            {
                if (q == head)
                {
                    head = head->next;
                    delete q;
                    q = head;
                }
                else
                {
                    preNode->next = q->next;
                    delete q;
                    q = preNode->next;
                }
            }
            else
            {
                preNode = q;
                q=q->next;
            }
        }
     return head;
    }