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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主要思路：
   
 首先定义两个指针，当前指针p和前一指针pre，用p指针的数值和pre的数值进行比较，如果相等，p地址删除，p指针前移，pre指针不变，如果不相等，则p和pre指针共同前移。

/**
* 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) {
if(head==NULL || head->next==NULL) return head;
ListNode* pre=head,*p=head->next;

while(pre!=NULL && p!=NULL)
{
if(p->val==pre->val)
{
ListNode* temp;
temp=p;
pre->next=p->next;
p=p->next;
free(temp);
}
else{
p=p->next;
pre=pre->next;
}
}
return head;
}
};

/**
* 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* now,*pre,*temp;;
if(!head || !head->next)
return head;
pre=head;
while(pre!=NULL && pre->next!=NULL)
{
if(pre->val==pre->next->val)
{
pre->next=pre->next->next;
}
else
{
pre=pre->next;
}
}
return head;
}
};