要用到两个指针遍历，一个是cur遍历老链表，一个是p用来做结果链表不断往里添加。

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* deleteDuplicates(ListNode* head) {
        if(!head || !head->next) return head;
        ListNode* dumpy = new ListNode(-1);
        ListNode* cur = head;
        dumpy->next = head;
        ListNode* p = dumpy;
        map<int,int> mp;
        while(cur){
            if(mp[cur->val] == 0){
                mp[cur->val] = 1;
                p->next = new ListNode(cur->val);//这里不能写成p->next = cur ;
                p = p->next;
            }
            cur = cur->next;
        }
        return dumpy->next;
    }
};