Remove Duplicates from Sorted List I & II

Title:

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

Title:

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.

class Solution{
    public:
ListNode *deleteDuplicates(ListNode *head) {
        if(head == NULL || head->next == NULL){
            return head;
        }
        ListNode *p = new ListNode(-1);
        p->next = head;
        ListNode *cur = p, *pre = head;
        while(pre != NULL){
            bool isDupli = false;
            while(pre->next != NULL && pre->val == pre->next->val){
                isDupli = true;
                pre = pre->next;
            }
            if(isDupli){
                pre = pre->next;
                continue;
             
            }
            cur->next = pre;
            cur = cur->next;
            pre = pre->next;
            
        }
        cur->next = pre;
        return p->next;
    }
};