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) {
        if(head==NULL)
            return NULL;
        ListNode *p=head;
        ListNode *pre=head;
        while(p)
        {
            if(p->val==val)
            {
                if(p==head)
                {
                    head=p->next;
                    p->next=NULL;
                    delete p;
                    pre=head;
                    p=head;
                }
                else
                {
                    pre->next=p->next;
                    p->next=NULL;
                    delete p;
                    p=pre->next;
                }
            }
            else
            {
                pre=p;
                p=p->next;
            }
        }
        return head;
    }
};