Partition a Linked list, pivoting around a value

LinkedPartition.cpp

ListNode* partition(ListNode* head, int x) {
        if (head == nullptr || head->next == nullptr)
            return head;
        ListNode l; ListNode *a = &l;
        ListNode r; ListNode *b = &r;
        ListNode* p = head;
        while (p != nullptr) {
            ListNode* t = p;
            p = p->next;
            t->next = nullptr;
            if (t->val < x) {
                a->next = t;
                a = a->next;
            } else {
                b->next = t;
                b = b->next;
            }
        }
        a->next = r.next;
        return l.next;
    }