ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {    if (l1 == nullptr){        return l2;    }        if (l2 == nullptr){        return l1;    }        auto head = l1;    auto otherHead = l2;    if (l2->val < l1->val){        head = l2;        otherHead = l1;    }        auto it1 = head;    auto it2 = otherHead;    while (it2 != nullptr){        if (it1->next == nullptr){            it1->next = it2;            it2 = it2->next;            break;        }        else if (it2->val > it1->next->val){            it1 = it1->next;        }        else{            auto tmp = it2;            it2 = it2->next;            tmp->next = it1->next;            it1->next = tmp;            it1 = it1->next;        }    }    return head;}