21.cpp

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode *head=new ListNode(0),*prev=head;
         if(l1==nullptr){return l2;}
         else if(l2==nullptr){return l1;}
         else{
            while(l1&&l2){
                if(l1->val<l2->val){
                    prev->next=l1;
                    l1=l1->next;
                }else{
                    prev->next=l2;
                    l2=l2->next;
                }
                prev=prev->next;
            }
         }
         prev->next=l1?l1:l2;
         return head->next;
    }
};