Remove Duplicates from Sorted List II

Given a sorted linked list, delete all nodes that have duplicate numbers, leaving only distinct numbers from the original list.

For example,
Given 1->2->3->3->4->4->5, return 1->2->5.
Given 1->1->1->2->3, return 2->3.

 

ListNode *deleteDuplicates(ListNode *head) {
    // Start typing your C/C++ solution below
    // DO NOT write int main() function
    if(head==NULL || head->next==NULL)
        return head;
    ListNode * p = head;
     8     while(p->next!=NULL && p->val == p->next->val){
        p=p->next;
    }

    if(p!=head){
        while(head!=p->next)
        {
            ListNode * tmp = head;
            head=head->next;
            free(tmp);
        }

        return deleteDuplicates(p->next);
    }

    head->next=deleteDuplicates(head->next);
    return head;
}