lt59.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* list1, ListNode* list2) {
        
        if(list1==NULL && list2==NULL){
            return nullptr;
            
        }
        if(list1==NULL){
            return list2;
        }
        if(list2==NULL){
            return list1;
        }
        ListNode* dummyhead= new ListNode(-1);
        ListNode* current= dummyhead;
        while(list1!=NULL && list2!=NULL){
            if(list1->val<list2->val){
                current->next= list1;
                list1=list1->next;
                current=current->next;
            }
            else{
                current->next= list2;
                list2= list2->next;
                current= current->next;
            }
        }

        if(list1!=NULL){
            current->next=list1;
            current=current->next;
        }else{
            current->next=list2;
            current=current->next;
        }

        return dummyhead->next;
    }
};