我不知道如何在c++中使用链表按从a到z的顺序显示字母表

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

ListNode* head = NULL;

void insertNode(char c) {
    ListNode* newNode = new ListNode;
    newNode->data = c;
    newNode->next = NULL;

    if (head == NULL) {
        head = newNode;
    } else {
        ListNode* current = head;
        ListNode* previous = NULL;

        while (current != NULL && current->data < c) {
            previous = current;
            current = current->next;
        }

        if (previous == NULL) {
            newNode->next = head;
            head = newNode;
        } else {
            previous->next = newNode;
            newNode->next = current;
        }
    }
}

void displayAlphabet() {
    ListNode* current = head;

    while (current != NULL) {
        cout << current->data << " ";
        current = current->next;
    }

    cout << endl;
}

int main() {
    insertNode('a');
    insertNode('c');
    insertNode('b');
    insertNode('e');
    insertNode('d');

    displayAlphabet();

    return 0;
}