Remove Linked List Elements

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
ListNode* removeElements(ListNode* head, int val) {
ListNode *currNode,*preNode,*p;
p=head;
if(head==NULL)
return NULL;
if(head->val==val)
{
while(head->val==val)
{
if(head->next)
head=head->next;
else
return NULL;
}
}
while(p->next)
{
preNode=p;
p=p->next;
if(p->val==val)
{
preNode->next=p->next;
p=preNode;              //此处要给p赋值回到前一个节点处
}
}
if(p->val==val)
{
preNode->next=NULL;
}
return head;
}
};