[LeetCode] Remove Duplicate from Sorted Linkded 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

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode *deleteDuplicates(ListNode *head) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        if(head==NULL)
            return head;
        
        ListNode *p1=NULL;
        ListNode *p2=head;
        
        while(p2!=NULL)
        {
            if(p2->next==NULL || (p2->next!=NULL && p2->val!=p2->next->val) )
            {
                if(p1==NULL)
                {
                    p1=new ListNode(0);
                    p1=p2;
                    head=p1;
                }
                else{
                    p1->next=p2;
                    p1=p2;
                }
        
                p2=p2->next;        
                //if(p2!=NULL && p2->next!=NULL)
                  //  p1->next=NULL;
        
                continue;
            }        
            
            int dup=p2->val;
            while(p2!=NULL && p2->val==dup)
            {
                if(p1!=NULL)
                    p1->next=NULL;
                ListNode *tmp=p2;
                p2=p2->next;
                delete tmp;
            }
        }
        
        if(p1==NULL) return NULL;
        
        return head;
    }
};