LeetCode-Remove Duplicates from Sorted List II

 1 /**
 2  * Definition for singly-linked list.
 3  * struct ListNode {
 4  *     int val;
 5  *     struct ListNode *next;
 6  * };
 7  */
 8 struct ListNode* deleteDuplicates(struct ListNode* head) {
 9     if (!head) return NULL;
10     if (!head->next) return head;
11     
12     struct ListNode *p = NULL;
13     struct ListNode *q = NULL;
14     struct ListNode *r = NULL;
15     struct ListNode *newHead = NULL;//newHead denotes the new list
16     int flag = 0, isHead = 0;//isHead=0 denotes there is no head in newHead
17     p = head, q = p->next;//p denotes the current node,q denotes the next node 
18     
19     flag = 0;//there are no duplications
20     while (q)
21     {
22         if (p->val != q->val && flag == 0)//put p in new list 
23         {
24             if (!isHead)
25             {
26                 newHead = r = p;
27                 isHead = 1;//there is already head in newHead
28                 r->next = NULL;                    
29             }
30             else
31             {
32                 r->next = p;
33                 r = p;        
34                 r->next = NULL;
35             }
36         }
37         else if (p->val != q->val && flag == 1)//delete p
38             flag = 0;
39         else if (p->val == q->val)//delete p
40             flag = 1;
41             
42         p = q;
43         q = q->next;
44     }
45     
46     if (flag == 1)
47     {
48         if (!isHead)
49             newHead == NULL;
50         else
51             r->next = NULL;
52     }
53     else
54     {
55         if (!isHead)
56         {
57             newHead = p;
58             newHead->next = NULL;
59         }
60         else
61         {
62             r->next = p;
63             r = p;
64             r->next = NULL;
65         }
66     }
67     
68     return newHead;
69 }