1，把比X小的节点放在一个链表，把大于X的节点放在另外一个链表，
2，把小的链表和大的链表连接起来
3，注意后面链表的末尾记得加NULL

struct ListNode* partition(struct ListNode* head, int x) {
        struct ListNode *d1, *d2, *last1, *last2;
        d1 = malloc(sizeof(struct ListNode));
        d2 = malloc(sizeof(struct ListNode));
        d1->next = d2->next = NULL;
        last1 = d1;
        last2 = d2;

        while(head){
            if(head->val < x){
                last1->next = head;
                last1 = last1->next;    
            }else{
                last2->next = head;
                last2 = last2->next;
            }

            head = head->next;
        }
        last1->next = d2->next;
        last2->next = NULL;//!!!!!!!!!!!!!!!!!!!!


        return d1->next;

}