合并两个排序的链表

class Solution {
public:
    ListNode* merge(ListNode* l1, ListNode* l2) {
        ListNode* dummy=new ListNode(-1),*tail=dummy;
        while(l1&&l2)
        {
            if(l1->val>l2->val)
            {
                tail->next=l2;
                l2=l2->next;
            }
            else
            {
                tail->next=l1;
                l1=l1->next;
            }
            tail=tail->next;
            
        }
        tail->next=l1?l1:l2;
        return  dummy->next;
    }
};