25合并两个排序的链表

class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode* p = l1;
        ListNode* q = l2;
        ListNode* r = new ListNode;
        ListNode* t = r;
        while (p != NULL && q != NULL) {
            if (p->val > q->val) {
                r ->next = q;
                q = q->next;
                r = r->next;
            }
            else {
                r->next = p;
                p = p->next;
                r = r->next;
            }
        }
        if (p != NULL) {
            r->next = p;
        }
        if (q != NULL) {
            r->next = q;
        }
        return t->next;
    }
};