class Partition {
public:
    ListNode* partition(ListNode* pHead, int x) 
    {
        ListNode *greatertail , *greaterhead, *lesshead, *lesstail;
        greaterhead = greatertail = (struct ListNode*)malloc(sizeof(struct ListNode));    //哨兵位
        lesshead = lesstail = (struct ListNode*)malloc(sizeof(struct ListNode));    //哨兵位
        greaterhead -> next = NULL;
        lesshead ->next = NULL;
        struct ListNode* cur = pHead;    // 工具指针
        while(cur)
        {
            if(cur -> val < x)
            {
                lesstail -> next = cur;
                lesstail = lesstail->next;
            }
            else
            {
                greatertail -> next = cur;
                greatertail = greatertail->next;
            }
                cur = cur->next;
        }
        lesstail->next = greaterhead->next;
        greatertail->next = NULL;   
        struct ListNode* head = lesshead->next;
        free(greaterhead);
        free(lesshead);
        return head;
    }
};