Leetcode：delete all duplicate nodes

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.

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1. Using dummy node as the head may be deleted.
2. note the following two cases:

•      define a int val to guarantee all duplicates are deleted. 
•      define a pointer p to cover two nodes, note the boundary check

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//code

class Solution {

public:

       ListNode* deleteDuplicates(ListNode* head) {

              //check input

              if (head == NULL)return head;

              ListNode *dum = new ListNode(-1), *tmp = head, *p = head;

              dum->next = head;

              p = dum;

              int val = INT_MAX;

              while (p->next->next != NULL){

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

                           val = p->next->val;

                           tmp = p->next;

                           p->next = tmp->next;

                           delete tmp;

                     }

                     else p = p->next;

              }

              //delete left val

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

                     tmp = p->next;

                     p->next = tmp->next;

                     delete tmp;

              }

              head = dum->next;

              delete dum;

              return head;

       }

};