链表（Linked List）的C语言实现

链表中的各对象按线性顺序排列，而其顺序是由各个对象里的指针所决定的。
链表有多种形式，它可以是单链接的或者双链接的，可以是已排序的或未排序的，可以是循环的或非循环的。
定义的结构体以及函数如下：

typedef struct SINGLE_LINKED_LIST{    Node * head;    Node * tail;} SLL;typedef struct CIRCULAR_SINGLE_LINKED_LIST{    Node * head;    Node * tail;} CSLL;typedef struct DOUBLE_LINKED_LIST{    Node * head;    Node * tail;} DLL;typedef struct CIRCULAR_DOUBLE_LINKED_LIST_WITH_SENTINEL{    Node * nil;} CDLLS;void linked_list_traverse(Node * L);void linked_list_free(Node* L);Node* single_linked_list_insert(SLL * L, item_t item);Node* single_linked_list_search(SLL * L, item_t item);int single_linked_list_delete(SLL * L, Node * node);Node* circular_single_linked_list_insert(CSLL * L, item_t item);Node* circular_single_linked_list_search(CSLL * L, item_t item);int circular_single_linked_list_delete(CSLL * L, Node * node);Node* double_linked_list_insert(DLL * L, item_t  item);Node* double_linked_list_search(DLL * L, item_t item);int double_linked_list_delete(DLL * L, Node * node);CDLLS* circular_double_linked_list_with_sentinel_init();Node* circular_double_linked_list_with_sentinel_insert(CDLLS * L, item_t item);Node* circular_double_linked_list_with_sentinel_search(CDLLS * L, item_t item);int circular_double_linked_list_with_sentinel_delete(CDLLS * L, Node * node);

几个公共函数如下：

//functions for all kind of linked listvoid linked_list_traverse(Node* L) {    if (L == NULL) {        printf("Empty Linked List.\n");    }    Node * pos = L;    while (pos != NULL) {        printf("%2d is in location:%9p. prev is %9p. next is %9p.\n", pos->item.key, \            pos, pos->prev, pos->next);        pos = pos->next;        if (pos == L) return;    }}void linked_list_free(Node* L) {    Node * pos = L;    Node * f = L;    while (pos != NULL && pos != L) {        f = pos;        pos = pos->next;        free(f);    }}//-----------------------------------------------------------------------

单链接的未排序非循环链表可以如如下实现：

//functions for single linked listNode* single_linked_list_insert(SLL * L, item_t item) {    Node * node = (Node*)malloc(sizeof(Node));    node->item = item;    node->prev = NULL;    node->next = L->head;    if (L->tail == NULL)        L->tail = node;    L->head = node;    return node;}Node* single_linked_list_search(SLL * L, item_t item) {    if (L->head == NULL) {        fprintf(stderr, "The single linked list is empty.\n");        return NULL;    }    Node * pos = L->head;    while (pos != NULL) {        if (item.key == pos->item.key)            return pos;        pos = pos->next;    }    fprintf(stderr, "The item cannot be found.\n");    return NULL;}int single_linked_list_delete(SLL * L, Node * node) {    if (L->head == NULL) {        fprintf(stderr, "The single linked list is empty.\n");        return 0;    }    if (node == NULL) {        fprintf(stderr, "The node is NULL.\n");        return 0;    }    if (L->head == node) {        L->head = node->next;        if (L->head == NULL)            L->tail = NULL;        return 1;    }    Node * pos = L->head;    while (pos != NULL && pos->next != node) {        pos = pos->next;    }    if (pos->next = node) {        if (node == L->tail)            L->tail = pos;        pos->next = pos->next->next;        return 1;    }    fprintf(stderr, "No such node in linked list.\n");    return 0;}int single_linked_list_reverse(SLL * L) {    if (L->head == NULL) {        fprintf(stderr, "The single linked list is empty.\n");        return 0;    }    L->tail = L->head;    Node *pre, *current, *next;    pre = L->head;    current = L->head->next;    L->head->next = NULL;    next = NULL;    while (current != NULL) {        next = current->next;        current->next = pre;        pre = current;        current = next;    }    L->head = pre;    return 1;}//------------------------------------------------------------------------

各个函数的功能均可见其函数名与返回值。使用SLL时动态分配内存即可，如：

void test_for_single_linked_list() {    SLL * L = (SLL*)malloc(sizeof(SLL));    printf("%p\n", L->head);    item_t item = {10, NULL};    Node * node;    for (int i = 0; i < 10; i++) {        item.key = i + 10;        single_linked_list_insert(L, item);    }    printf("SLL head is in location:%p\n", L->head);    linked_list_traverse(L->head);    single_linked_list_reverse(L);    linked_list_traverse(L->head);    for (int i = -1; i <= 10; i++) {        item.key = i + 10;        node = single_linked_list_search(L, item);        if (node != NULL)            printf("Node %d in location %p delete :%d\n", node->item.key, node, \                single_linked_list_delete(L, node));        //printf("SLL tail is in location:%p\n", L->tail);    }    printf("---------------------------------------------\n");    for (int i = 0; i < 10; i++) {        item.key = i + 10;        single_linked_list_insert(L, item);    }    printf("SLL head is in location:%p\n", L->head);    linked_list_traverse(L->head);    for (int i = 11; i >= -1; i--) {        item.key = i + 10;        node = single_linked_list_search(L, item);        if (node != NULL)            printf("Node %d in location %p delete :%d\n", node->item.key, node, \                single_linked_list_delete(L, node));        //printf("SLL tail is in location:%p\n", L->tail);    }    linked_list_free(L->head);    free(L);}

双链接的未排序非循环链表可如如下实现：

//functions for double linked listNode* double_linked_list_insert(DLL * L, item_t item) {    Node * node = (Node*)malloc(sizeof(Node));    node->prev = NULL;    node->item = item;    node->next = L->head;    if (L->head != NULL)        L->head->prev = node;    else        L->tail = node;    L->head = node;    return node;}Node* double_linked_list_search(DLL * L, item_t item) {    if (L->head == NULL) {        fprintf(stderr, "The double linked list is empty.\n");        return NULL;    }    Node * pos = L->head;    while (pos != NULL) {        if (pos->item.key == item.key)            return pos;        pos = pos->next;    }    fprintf(stderr, "The item cannot be found.\n");    return NULL;}int double_linked_list_delete(DLL * L, Node * node) {    if (L->head == NULL) {        fprintf(stderr, "The double linked list is empty.\n");        return 0;    }    if (node == NULL) {        fprintf(stderr, "The node is NULL.\n");        return 0;    }    if (node == L->tail)        L->tail = node->prev;    if (node->prev != NULL) {        node->prev->next = node->next;    } else {        L->head = node->next;    }    if (node->next != NULL) {        node->next->prev = node->prev;    }    return 1;}//----------------------------------------------------------------------------

注意要点与单链接的未排序非循环链表相同。测试可如下：

void test_for_double_linked_list() {    DLL * L = (DLL*)malloc(sizeof(DLL));    printf("%p\n", L->head);    item_t item = {10, NULL};    Node * node;    for (int i = 0; i < 10; i++) {        item.key = i + 10;        double_linked_list_insert(L, item);    }    printf("DLL head is in location:%p\n", L->head);    linked_list_traverse(L->head);    for (int i = -1; i <= 10; i++) {        item.key = i + 10;        node = double_linked_list_search(L, item);        if (node != NULL)            printf("Node %d in location %p delete :%d\n", node->item.key, node, \                double_linked_list_delete(L, node));        //printf("DLL tail is in location:%p\n", L->tail);    }    printf("---------------------------------------------\n");    for (int i = 0; i < 10; i++) {        item.key = i + 10;        double_linked_list_insert(L, item);    }    printf("DLL head is in location:%p\n", L->head);    linked_list_traverse(L->head);    for (int i = 11; i >= -1; i--) {        item.key = i + 10;        node = double_linked_list_search(L, item);        if (node != NULL)            printf("Node %d in location %p delete :%d\n", node->item.key, node, \                double_linked_list_delete(L, node));        //printf("DLL tail is in location:%p\n", L->tail);    }    linked_list_free(L->head);    free(L);}

单链接未排序的循环链表可如如下实现：

//functinos for circular single linked listNode* circular_single_linked_list_insert(CSLL * L, item_t item) {    Node * node = (Node*)malloc(sizeof(Node));    node->item = item;    node->prev = NULL;    if (L->head == NULL) {        L->head = node;        L->tail = node;        node->next = node;    } else {        node->next = L->head;        L->head = node;        L->tail->next = node;    }    return node;}Node* circular_single_linked_list_search(CSLL * L, item_t item) {    if (L->head == NULL) {        fprintf(stderr, "The circular single linked list is empty.\n");        return NULL;    }    Node * pos = L->head;    if (pos->item.key == item.key)        return pos;    else        pos = pos->next;    while (pos->item.key != item.key && pos != L->head) {        pos = pos->next;    }    if (pos != L->head)        return pos;    fprintf(stderr, "The item cannot be found.\n");    return NULL;}int circular_single_linked_list_delete(CSLL * L, Node * node) {    if (L->head == NULL) {        fprintf(stderr, "The circular single linked list is empty.\n");        return 0;    }    if (node == NULL) {        fprintf(stderr, "The node is NULL.\n");        return 0;    }    if (node == L->head && node == L->tail) {        L->head = NULL;        L->tail = NULL;        return 1;    } else if (node == L->head) {        L->head = node->next;        L->tail->next = L->head;        return 1;    }    Node * pos = L->head;    while (pos->next != node && pos != L->tail) {        pos = pos->next;    }    if (pos->next == node) {        if (node == L->tail) {            L->tail = pos;        }        pos->next = node->next;        return 1;    }    fprintf(stderr, "No such node in linked list.\n");    return 0;}//------------------------------------------------------------------------

测试可如下：

void test_for_circular_single_linked_list() {    CSLL * L = (CSLL*)malloc(sizeof(CSLL));    printf("%p\n", L->head);    item_t item = {10, NULL};    Node * node;    for (int i = 0; i < 10; i++) {        item.key = i + 10;        circular_single_linked_list_insert(L, item);    }    printf("CSLL head is in location:%p\n", L->head);    linked_list_traverse(L->head);    for (int i = -1; i <= 10; i++) {        item.key = i + 10;        node = circular_single_linked_list_search(L, item);        if (node != NULL)            printf("Node %d in location %p delete :%d\n", node->item.key, node, \                circular_single_linked_list_delete(L, node));        //linked_list_traverse(L->head);    }    printf("---------------------------------------------\n");    for (int i = 0; i < 10; i++) {        item.key = i + 10;        circular_single_linked_list_insert(L, item);    }    printf("CSLL head is in location:%p\n", L->head);    linked_list_traverse(L->head);    for (int i = 11; i >= -1; i--) {        item.key = i + 10;        node = circular_single_linked_list_search(L, item);        if (node != NULL)            printf("Node %d in location %p delete :%d\n", node->item.key, node, \                circular_single_linked_list_delete(L, node));    }    linked_list_free(L->head);    free(L);}

有哨兵的双链接未排序循环链表可如如下实现：

//functions for cicular double linked list with sentinelCDLLS* circular_double_linked_list_with_sentinel_init() {    CDLLS * L = (CDLLS*)malloc(sizeof(CDLLS));    L->nil = (Node*)malloc(sizeof(Node));    L->nil->next = L->nil;    L->nil->prev = L->nil;    return L;}Node* circular_double_linked_list_with_sentinel_insert(CDLLS * L, item_t item) {    Node * node = (Node*)malloc(sizeof(Node));    node->item = item;    node->next = L->nil->next;    L->nil->next->prev = node;    node->prev = L->nil;    L->nil->next = node;    return node;}Node* circular_double_linked_list_with_sentinel_search(CDLLS * L, item_t item) {    if (L->nil->next == L->nil) {        fprintf(stderr, "The circular double linked list is empty.\n");        return NULL;    }    Node* pos = L->nil->next;    while (pos != L->nil && pos->item.key != item.key) {        pos = pos->next;    }    if (pos == L->nil) {        fprintf(stderr, "The item cannot be found.\n");        return NULL;    } else {        return pos;    }}int circular_double_linked_list_with_sentinel_delete(CDLLS * L, Node * node) {    if (L->nil->next == L->nil) {        fprintf(stderr, "The circular double linked list is empty.\n");        return 0;    }    if (node == NULL) {        fprintf(stderr, "The node is NULL.\n");        return 0;    }    node->prev->next = node->next;    node->next->prev = node->prev;    return 1;}//--------------------------------------------------------------------------

测试可如下：

void test_for_circular_double_linked_list() {    CDLLS * L = circular_double_linked_list_with_sentinel_init();    printf("%p %p %p %p\n", L, L->nil, L->nil->prev, L->nil->prev);    item_t item = {10, NULL};    Node * node;    for (int i = 0; i < 10; i++) {        item.key = i + 10;        circular_double_linked_list_with_sentinel_insert(L, item);    }    printf("CDLLS nil is in location:%p\n", L->nil);    linked_list_traverse(L->nil);    for (int i = -1; i <= 10; i++) {        item.key = i + 10;        node = circular_double_linked_list_with_sentinel_search(L, item);        if (node != NULL)            printf("Node %d in location %p delete :%d\n", node->item.key, node, \                circular_double_linked_list_with_sentinel_delete(L, node));    }    printf("---------------------------------------------\n");    for (int i = 0; i < 10; i++) {        item.key = i + 10;        circular_double_linked_list_with_sentinel_insert(L, item);    }    printf("CDLLS nil is in location:%p\n", L->nil);    linked_list_traverse(L->nil);    for (int i = 11; i >= -1; i--) {        item.key = i + 10;        node = circular_double_linked_list_with_sentinel_search(L, item);        if (node != NULL)            printf("Node %d in location %p delete :%d\n", node->item.key, node, \                circular_double_linked_list_with_sentinel_delete(L, node));    }    linked_list_free(L->nil);    free(L);}

可以将其看为含有哨兵的一个循环的圆圈，这样理解比较方便。