Remove Duplicates from Sorted List

题目：

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.

解析：

开始因为疏忽出现了两次runtime error情况，对于链表的问题一般就是如下情况

1. NULL->next

2. 指针是NULL，还用指针的val，p = NULL，p->val

/**
 * 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 *p = head;
        while(p->next)
        {
            if(p->val == p->next->val)
                p->next = p->next->next;
            else
                p = p->next;
        }
        return head;
    }
};