面试题 02.01：移除重复节点

输入：[1, 2, 3, 3, 2, 1]
 输出：[1, 2, 3]

输入：[1, 1, 1, 1, 2]
 输出：[1, 2]

class Solution {
public:
    ListNode* removeDuplicateNodes(ListNode* head) {
        unordered_set<int> set;
        ListNode* pre=nullptr;
        ListNode* cur=head;
        while(cur){
            if(set.count(cur->val)){
                pre->next=cur->next;
                cur=cur->next;
            }
            else{
                set.insert(cur->val);
                pre=cur;
                cur=cur->next;
            }
        }
        return head;
    }
};