创建一个虚拟头结点  从虚拟头结点去遍历  

public class Solution
{
    public ListNode RemoveElements(ListNode head, int val)
    {
        ListNode dummyHead = new ListNode(-1);
        dummyHead.next = head;
        ListNode prev = dummyHead;

        while (prev.next != null)
        {
            if (prev.next.val == val)
            {
                prev.next = prev.next.next;
            }
            else
            {
                prev = prev.next;
            }
        }
        return dummyHead.next;

    }
}