LeetCode Merge Two Sorted Lists

class Solution {public:    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {        ListNode *ans=new ListNode(-1);//哨位节点，尾差；        ListNode * tail=ans;        while (l1&&l2) {            if (l1->val<l2->val) {//将l1插入                tail->next=new ListNode(l1->val);                tail=tail->next;                l1=l1->next;            }else{                tail->next=new ListNode(l2->val);                tail=tail->next;                l2=l2->next;            }        }        while (l1) {            tail->next=new ListNode(l1->val);            tail=tail->next;            l1=l1->next;        }        while (l2) {            tail->next=new ListNode(l2->val);            tail=tail->next;            l2=l2->next;        }        ans=ans->next;        return ans;            }};