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.

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This problem doesn't need a dummy in head, as the first one can be functioned as a dummy due to that the first node is always kept in the process. 

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//code

class Solution {

public:

       ListNode* deleteDuplicates(ListNode* head) {

              //check input

              if (head == NULL)return head;

              ListNode *tmp = head, *p = head;

              while (p->next != NULL){

                     if (p->next->val == p->val){

                           tmp = p->next;

                           p->next = tmp->next;

                           delete tmp;

                     }

                     else p = p->next;

              }

              return head;

       }

};