如何在C中的O(1)时间内合并两个链表？

struct ListNode {
    int data;
    struct ListNode* next;
};

struct ListNode* list1 = NULL;
struct ListNode* list2 = NULL;

void append(struct ListNode** head, int data) {
    struct ListNode* newNode = (struct ListNode*)malloc(sizeof(struct ListNode));
    newNode->data = data;
    newNode->next = NULL;

    if (*head == NULL) {
        *head = newNode;
    } else {
        struct ListNode* current = *head;
        while (current->next != NULL) {
            current = current->next;
        }
        current->next = newNode;
    }
}

struct ListNode* mergeLists(struct ListNode* list1, struct ListNode* list2) {
    if (list1 == NULL) {
        return list2;
    }
    if (list2 == NULL) {
        return list1;
    }

    struct ListNode* mergedList = list1;
    while (list1->next != NULL) {
        list1 = list1->next;
    }
    list1->next = list2;

    return mergedList;
}

append(&list1, 1);
append(&list1, 2);
append(&list1, 3);

append(&list2, 4);
append(&list2, 5);
append(&list2, 6);

struct ListNode* mergedList = mergeLists(list1, list2);