/**
 * 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 *head = new ListNode;
        ListNode *tmp = head;
        while(list1 != nullptr || list2 != nullptr)
        {
            if((list1 != nullptr && list2 != nullptr && list1->val <= list2->val)
                ||(list1 != nullptr && list2 == nullptr))
            {
                tmp->next = list1;
                tmp = tmp->next;
                list1 = list1->next;
            }
            if((list1 != nullptr && list2 != nullptr && list1->val > list2->val)
                ||(list1 == nullptr && list2 != nullptr))
            {
                tmp->next = list2;
                tmp = tmp->next;
                list2 = list2->next;
            }
        }
        return head->next;
    }
};