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* list1, ListNode* list2) {
        
        ListNode * temp = nullptr;
        ListNode * temp2 = nullptr;
        ListNode * answer_head = nullptr;
        
        while (list1 != nullptr && list2 != nullptr){
                
            if(list1->val <= list2->val) {
                temp = new ListNode(list1->val);
                list1 = list1 -> next;
            }
            else {
                temp = new ListNode(list2->val);
                list2 = list2 -> next;
            }
            
            
            if (temp2 == nullptr){
                temp2 = temp;
                answer_head = temp2;
                continue;
            }
            
            temp2->next = temp;
            temp2 = temp2-> next;
            
      
        }
            
        while (list1 != nullptr){
            temp = new ListNode(list1->val);
            list1 = list1->next;
            
            if (temp2 == nullptr){
                temp2 = temp;
                answer_head = temp2;
                continue;
            }
            
            temp2->next = temp;
            temp2 = temp2-> next;
            
           
        }
        
        while (list2 != nullptr){
            temp = new ListNode(list2->val);
            list2 = list2->next;
            
            if (temp2 == nullptr){
                temp2 = temp;
                answer_head = temp2;
                continue;
            }
            
            temp2->next = temp;
            temp2 = temp2-> next;
            

        }
        return answer_head;
    }
};