LeetCode-Remove Duplicates from Sorted List

Given a sorted linked list, delete all duplicates such that each element appear only once.

For example,
Given 1->1->2, return 1->2.
Given 1->1->2->3->3, return 1->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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if(head==NULL)return head;
        if(head->next==NULL)return head;
        int p1;
        ListNode* ptr=head,*ptr2;
        p1=head->val;
        ptr2=head;
        ptr=head->next;
        while(ptr!=NULL){
            if(ptr->val!=p1){
                ptr2->next=ptr;
                ptr2=ptr;
                ptr=ptr->next;
                p1=ptr2->val;
            }
            else{
                ptr=ptr->next;
            }
        }
        ptr2->next=NULL;
        return head;
    }
};