merge_LL.cpp

class Solution {
public:
    ListNode* mergeTwoLists(ListNode* list1, ListNode* list2) {
       
       ListNode *dummy=NULL;
        if(list1==NULL) return list2;
        if(list2==NULL) return list1;
        if(list1->val > list2->val) std:: swap(list1,list2);
       ListNode *dhead=list1;
        while(list1!=NULL && list2!=NULL){
           dummy=NULL;
            while(list1!=NULL && list1->val <= list2->val){
                dummy=list1;
                list1=list1->next;
            }
            dummy->next=list2;
            std::swap(list1,list2);
        }
        return dhead;
    }
};