83. Remove Duplicates from Sorted List

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

Example 1:

Input: 1->1->2
Output: 1->2

Example 2:

Input: 1->1->2->3->3
Output: 1->2->3

my code:

/**
 * 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* dummy = new ListNode(0);
        dummy->next = head;
        ListNode* p = dummy;
        while (p->next != NULL && p->next->next != NULL) {
            int sameNum = p->next->val;   
            p = p->next;
            while (p->next!= NULL && p->next->val == sameNum) {
                if (p->next->next != NULL)
                    p->next = p->next->next;
                else 
                    p->next = NULL;
            }
        }
        return dummy->next;
    }
};

Runtime: 12 ms, faster than 36.37% of C++ online submissions for Remove Duplicates from Sorted List.