Funtcion Interface Definition

typedef int ElementType;

typedef struct _dnode {
    ElementType value;
    struct _dnode *previous;
    struct _dnode *next;
} DNode;

typedef DNode* DList;

DList initializeList();
void appendNode(DList list, ElementType value);
bool isEmpty(DList list);
void forwardTraverse(DList list);
void backwardTraverse(DList list);
DNode* findNode(DList list, ElementType value);
DNode* removeSingleNode(DList list, DNode* target);
void removeAllNodes(DList list, ElementType value);
void clearList(DList list);
void deallocateList(DList list);

Sample Test Program

#include <stdio.h>
#include <stdlib.h>
#include <stdbool.h>

typedef int ElementType;

typedef struct _dnode {
    ElementType value;
    struct _dnode *previous;
    struct _dnode *next;
} DNode;

typedef DNode* DList;

DList initializeList();
void appendNode(DList list, ElementType value);
bool isEmpty(DList list);
void forwardTraverse(DList list);
void backwardTraverse(DList list);
DNode* findNode(DList list, ElementType value);
DNode* removeSingleNode(DList list, DNode* target);
void removeAllNodes(DList list, ElementType value);
void clearList(DList list);
void deallocateList(DList list);

int main() {
    int input;
    DList myList = initializeList();
    
    scanf("%d", &input);
    while (input != 0) {
        appendNode(myList, input);
        scanf("%d", &input);
    }
    
    forwardTraverse(myList);
    backwardTraverse(myList);
    
    scanf("%d", &input);
    DNode *found = findNode(myList, input);
    if (found != NULL) {
        removeSingleNode(myList, found);
    }
    
    forwardTraverse(myList);
    backwardTraverse(myList);
    
    scanf("%d", &input);
    removeAllNodes(myList, input);
    
    forwardTraverse(myList);
    backwardTraverse(myList);
    
    deallocateList(myList);
    return 0;
}

/* Implementation goes here */

Implementation Code

DList initializeList() {
    DList head = (DList)malloc(sizeof(DNode));
    if (head == NULL) exit(EXIT_FAILURE);
    head->next = head;
    head->previous = head;
    return head;
}

void appendNode(DList list, ElementType value) {
    DNode* newNode = (DNode*)malloc(sizeof(DNode));
    newNode->value = value;
    DNode* tail = list->previous;
    
    newNode->previous = tail;
    tail->next = newNode;
    newNode->next = list;
    list->previous = newNode;
}

bool isEmpty(DList list) {
    return (list->next == list && list->previous == list);
}

void forwardTraverse(DList list) {
    if (isEmpty(list)) {
        printf("NULL");
        return;
    }
    
    DNode* current = list->next;
    while (current != list) {
        printf("%d ", current->value);
        current = current->next;
    }
    printf("\n");
}

void backwardTraverse(DList list) {
    if (isEmpty(list)) {
        printf("NULL");
        return;
    }
    
    DNode* current = list->previous;
    while (current != list) {
        printf("%d ", current->value);
        current = current->previous;
    }
    printf("\n");
}

DNode* findNode(DList list, ElementType value) {
    if (isEmpty(list)) return NULL;
    
    DNode* current = list->next;
    while (current != list) {
        if (current->value == value) return current;
        current = current->next;
    }
    return NULL;
}

DNode* removeSingleNode(DList list, DNode* target) {
    target->previous->next = target->next;
    target->next->previous = target->previous;
    DNode* nextNode = target->next;
    free(target);
    return nextNode;
}

void removeAllNodes(DList list, ElementType value) {
    DNode* current = list->next;
    DNode* temp;
    
    while (current != list) {
        temp = current->next;
        if (current->value == value) {
            current->previous->next = current->next;
            current->next->previous = current->previous;
            free(current);
        }
        current = temp;
    }
}

void clearList(DList list) {
    DNode* current = list->next;
    DNode* temp;
    
    while (current != list) {
        temp = current->next;
        free(current);
        current = temp;
    }
    list->next = list;
    list->previous = list;
}

void deallocateList(DList list) {
    clearList(list);
    free(list);
}