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/**
 * 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) {

        cin.tie(nullptr)->sync_with_stdio(0);

        ListNode* dumyNode = new ListNode(0);
        ListNode* iterNode = dumyNode;

        while (list1 != nullptr && list2 != nullptr) {

            if (list1->val <= list2->val) {
                iterNode->next = list1;
                list1 = list1->next;
            } else {

                iterNode->next = list2;
                list2 = list2->next;
            }
            iterNode = iterNode->next;
        }

        while (list2 == nullptr && list1 != nullptr) {
            iterNode->next = list1;
            list1 = list1->next;
            iterNode = iterNode->next;
        }
        while (list1 == nullptr && list2 != nullptr) {

            iterNode->next = list2;
            list2 = list2->next;
            iterNode = iterNode->next;
        }

        return dumyNode->next;
    }
};