LeetCode 21 合并两个有序链表

我的

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        ListNode* res;
        if(l1==NULL){
            res=l2;
            return res;
        }
        else if(l2==NULL){
            res=l1;
            return res;
        }
        else if((l2->val)<(l1->val)){
            res=l2;
            
            l2=l2->next;
        }
        else{
            res=l1;
            
            l1=l1->next;
        }
        ListNode* ans=res;
        while(1){
            if(l1==NULL){
                res->next=l2;
                return ans;
            }
            else if(l2==NULL){
                res->next=l1;
                return ans;
            }
            else if((l2->val)<(l1->val)){
                res->next=l2;
                res=res->next;
                l2=l2->next;
            }
            else{
                res->next=l1;
                res=res->next;
                l1=l1->next;
            }
        }
        
        return ans;
    }
};