合并链表

ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
       
        if(l1 == NULL){
            if(l2 == NULL){
                return NULL;
            }else{
                return l2;
            }
        }
        if(l1 != NULL && l2 == NULL){
            return l1;
        }
        ListNode* head=NULL;
        ListNode* ans=head;
        while(l1!=NULL&&l2!=NULL){
            ListNode* curr=NULL;
            if(l1->val < l2->val){
                curr=new ListNode(l1->val);
                l1=l1->next;
            }else{
                curr=new ListNode(l2->val);
                l2=l2->next;
            }
            if(head == NULL){
                head=curr;
                ans=head;
            }else{
                head->next=curr;
                head=head->next;
            }
            
        }
        while(l1!=NULL){
            ListNode* curr=new ListNode(l1->val);
            head->next=curr;
            head=head->next;
            l1=l1->next;
        }
        while(l2!=NULL){
            ListNode* curr=new ListNode(l2->val);
            head->next=curr;
            head=head->next;
            l2=l2->next;
        }
        return ans;
    }