[LeetCode] 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.

首先我们把key设为非头节点的值，然后遍历链表，当节点值等于key时删除，不能于时更新key.

/**
 * 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) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (head == NULL)
            return NULL;
        
        ListNode *pPre = NULL;
        ListNode *p = head;
        int key = INT_MAX;
        
        while(p)
        {
            if (key != p->val)
            {
                key = p->val;
                pPre = p;
                p = p->next;
            }
            else
            {
                ListNode *pNext = p->next;
                if (pPre)
                    pPre->next = pNext;
                delete p;
                p = pNext;
            }
        }
        
        return head;
    }
};