用C语言的有序链表实现集合的并、交、差运算，集合中的元素限定为小写字母、

数字和下划线，每个元素长度不超过20个字符。

可以使用以下代码实现有序链表：

typedef struct node {
    char data[21];
    struct node *next;
} Node;

typedef struct {
    Node *head;
    int size;
} List;

void init(List *list) {
    list->head = NULL;
    list->size = 0;
}

void insert(List *list, char *data) {
    Node *node = (Node *)malloc(sizeof(Node));
    strcpy(node->data, data);
    node->next = NULL;

    if (list->head == NULL) {
        list->head = node;
    } else {
        Node *prev = NULL;
        Node *curr = list->head;
        while (curr != NULL && strcmp(curr->data, data) < 0) {
            prev = curr;
            curr = curr->next;
        }

        if (prev == NULL) {
            node->next = list->head;
            list->head = node;
        } else {
            node->next = prev->next;
            prev->next = node;
        }
    }

    list->size++;
}

void remove(List *list, char *data) {
    Node *prev = NULL;
    Node *curr = list->head;
    while (curr != NULL && strcmp(curr->data, data) != 0) {
        prev = curr;
        curr = curr->next;
    }

    if (curr != NULL) {
        if (prev == NULL) {
            list->head = curr->next;
        } else {
            prev->next = curr->next;
        }

        free(curr);
        list->size--;
    }
}

int contains(List *list, char *data) {
    Node *curr = list->head;
    while (curr != NULL && strcmp(curr->data, data) < 0) {
        curr = curr->next;
    }

    return curr != NULL && strcmp(curr->data, data) == 0;
}

void print(List *list) {
    Node *curr = list->head;
    while (curr != NULL) {
        printf("%s ", curr->data);
        curr = curr->next;
    }
    printf("\n");
}

void union(List *list1, List *list2, List *result) {
    Node *curr1 = list1->head;
    Node *curr2 = list2->head;
    while (curr1 != NULL && curr2 != NULL) {
        if (strcmp(curr1->data, curr2->data) < 0) {
            insert(result, curr1->data);
            curr1 = curr1->next;
        } else if (strcmp(curr1->data, curr2->data) > 0) {
            insert(result, curr2->data);
            curr2 = curr2->next;
        } else {
            insert(result, curr1->data);
            curr1 = curr1->next;
            curr2 = curr2->next;
        }
    }

    while (curr1 != NULL) {
        insert(result, curr1->data);
        curr1 = curr1->next;
    }

    while (curr2 != NULL) {
        insert(result, curr2->data);
        curr2 = curr2->next;
    }
}

void intersection(List *list1, List *list2, List *result) {
    Node *curr1 = list1->head;
    Node *curr2 = list2->head;
    while (curr1 != NULL && curr2 != NULL) {
        if (strcmp(curr1->data, curr2->data) < 0) {
            curr1 = curr1->next;
        } else if (strcmp(curr1->data, curr2->data) > 0) {
            curr2 = curr2->next;
        } else {
            insert(result, curr1->data);
            curr1 = curr1->next;
            curr2 = curr2->next;
        }
    }
}

void difference(List *list1, List *list2, List *result) {
    Node *curr1 = list1->head;
    Node *curr2 = list2->head;
    while (curr1 != NULL && curr2 != NULL) {
        if (strcmp(curr1->data, curr2->data) < 0) {
            insert(result, curr1->data);
            curr1 = curr1->next;
        } else if (strcmp(curr1->data, curr2->data) > 0) {
            curr2 = curr2->next;
        } else {
            curr1 = curr1->next;
            curr2 = curr2->next;
        }
    }

    while (curr1 != NULL) {
        insert(result, curr1->data);
        curr1 = curr1->next;
    }
}

int main() {
    List list1, list2, result;
    init(&list1);
    init(&list2);
    init(&result);

    insert(&list1, "a");
    insert(&list1, "b");
    insert(&list1, "c");
    insert(&list1, "d");
    insert(&list1, "e");

    insert(&list2, "c");
    insert(&list2, "d");
    insert(&list2, "e");
    insert(&list2, "f");
    insert(&list2, "g");

    printf("List 1: ");
    print(&list1);

    printf("List 2: ");
    print(&list2);

    union(&list1, &list2, &result);
    printf("Union: ");
    print(&result);

    init(&result);
    intersection(&list1, &list2, &result);
    printf("Intersection: ");
    print(&result);

    init(&result);
    difference(&list1, &list2, &result);
    printf("Difference: ");
    print(&result);

    return 0;
}

对于集合的并、交、差运算，可以分别使用 union、intersection、difference 函数实现。