合并两个排序的链表

输入两个递增排序的链表，合并这两个链表并使新链表中的结点仍然是按照递增排序的。

class Solution {
public:
    ListNode* merge(ListNode* l1, ListNode* l2) {
        auto dummy = new ListNode(-1);
        auto cur = dummy;

        while (l1 && l2) {
            if (l1->val <= l2->val) {
                cur->next = l1;
                l1 = l1->next;
            }
            else {
                cur->next = l2;
                l2 = l2->next;
            }
            cur = cur->next;
        }

        if (l2) cur->next = l2;
        if (l1) cur->next = l1;

        return dummy->next;
    }
};