# remove duplicates from sorted list I&II

## remove duplicates from sorted list I&II

### I easy problem

Go throhgh the whole list.Use two pointers,`p`point to head at first, `q` point to next of `p`. If `p` and `q` have different value, `p` and `q` move to their next. Otherwise, delete `q` and move `q` to its next.

`````` 1 /**
2  * Definition for singly-linked list.
3  * struct ListNode {
4  *     int val;
5  *     ListNode *next;
6  *     ListNode(int x) : val(x), next(NULL) {}
7  * };
8  */
9 class Solution {
10 public:
14         }
17         while ( q ) {
18             if ( p->val != q->val ) {
19                 p = q;
20                 q = p->next;
21             } else {
22                 p->next = q->next;
23                 delete(q);
24                 q = p->next;
25             }
26         }
28     }
29 };``````

### II is not very head

When we solve the linked list problem, add a node before the head is always convience way to avoid the special condition of the head. We add a node before the head, and use three pointer `pre``p``q`. if `p` and `q` have the same value, delete `q` and make a mark. Otherwise check the mark, if marked delete `p`.

`````` 1 /**
2  * Definition for singly-linked list.
3  * struct ListNode {
4  *     int val;
5  *     ListNode *next;
6  *     ListNode(int x) : val(x), next(NULL) {}
7  * };
8  */
9 class Solution {
10 public:
14         }
15         ListNode* newHead = new ListNode(-1);
20         int mark = 0;
21         while ( q ) {
22             if ( p->val == q->val ) {
23                 p->next = q->next;
24                 delete(q);
25                 q = p->next;
26                 mark = 1;
27             } else {
28                 if ( 1 == mark ) {
29                     mark = 0;
30                     delete(p);
31                     pre->next = q;
32                     p = q;
33                     q = q->next;
34                 } else {
35                     pre = p;
36                     p = p->next;
37                     q = q->next;
38                 }
39             }
40         }
41         if ( 1 == mark ) {
42             pre->next = q;
43             delete(p);
44         }
48     }
49 };``````