class Solution {

 public:

     ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {

         ListNode* res = new ListNode();

         ListNode* p = res;

         //都不为空

         while(l1 != nullptr && l2 != nullptr)

         {

             if(l1->val < l2->val)

             {

                 p->next = l1;

                 p = p->next;

                 l1 = l1->next;

             }

             else

             {

                 p->next = l2;

                 p = p->next;

                 l2 = l2->next;

             }

         }

         //至少一条为空

         if(l1 == nullptr)

             p->next = l2;

         else if (l2 == nullptr)

             p->next = l1;

         return res->next;

     }

 };