class Solution {public:    ListNode *partition(ListNode *head, int x) {        if (!head) return NULL;        if (!head->next) return head;        ListNode dum(-1); dum.next = head;        ListNode *pPre = &dum;        ListNode *plast = &dum;        ListNode *p = head;        while (p)        {            if (p->val < x)            {                if (pPre->next == p)                {                    pPre = p;                }                else                {                    ListNode *pPreNext = pPre->next;                    //    dis-link                    plast->next = p->next;                    //    Move ahead                                    pPre->next = p;                    p->next = pPreNext;                    pPre = p;                    //                    p = plast->next;                    continue;                }            }            plast = p;            p = plast->next;        }        return dum.next;    }};