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.

分析： two pointers，pre_p 和 p, 注意删除p与不删除p，pre_p与pre_p->next的更新。

代码：

class Solution {
public:
    ListNode *deleteDuplicates(ListNode *head) {
        if(head == NULL || head->next == NULL) return head;
        
        for(ListNode * pre_p = head, * p = head->next; p != NULL; p = p->next){
            if(pre_p->val == p->val){
                pre_p->next = p->next;
            }else{
                pre_p = pre_p->next;
            }
        }
        
        return head;
    }
};