如何删除C中链表中的最后一个元素？

Question

我不知道如何在 c 中删除链表中的第一个和最后一个元素。下面是我的程序，它使用链表。我不知道如何实际删除最后一个元素，但我能找到它。该元素由一个整数和下一个指针组成。如果有人可以帮助我，将不胜感激。

struct ListNode{
    int value;
    struct ListNode *next;
};
typedef struct ListNode Link;

void deleteLast(Link *);
void printList(Link *);
void deleteFirst(Link *);

int main(){
    Link *myList;
 Link *curr, *newlink;
 int i;
    curr = myList;
    for(i = 0; i < 10; i++){

            newlink = (Link*) malloc(1*sizeof(Link));
            newlink->value = i*i;
            curr->next = newlink;
            curr = newlink;
    }

    curr->next = NULL;
    curr = myList->next;
    deleteFirst(curr);
    printList(curr);
    printf("\n");
}
void deleteLast(Link *head)
{
    Link *curr;
    curr = head->next;
    while (curr->next!=NULL)
    {
        curr = curr->next;
    }
     free(curr->next);

    curr->next = NULL;

}
void printList(Link *head){
    Link *curr;
    curr = head->next;

    printf("[");
    if(curr!=NULL){
        printf("%d",curr->value);
        curr = curr->next;
    }

    while(curr != NULL){
        printf(", %d", curr->value);
        curr = curr->next;
    }
    printf("]\n");

}
void deleteFirst(Link *head){
    Link *curr;
    curr = head->next;
    free(curr->value);
    free(curr->next);
    printf("%d\t",curr->value);
}

我尝试的任何方法都不起作用，请你帮帮我。

Answer 1

您的代码中有很多错误。

• 创建列表时：
  • 您不必强制转换 malloc 的返回值
  • 您正在执行curr->next = newlink; 其中 curr = myList 具有初始化值。您可以将循环更改为

  Link *myList = NULL;
  Link *curr, *newlink;
  int i;

  curr = myList;
  for(i = 0; i < 10; i++){

    newlink = malloc(1*sizeof(Link));
    newlink->value = i*i;
    if (curr != NULL)
      curr->next = newlink;
    else
      myList = newlink;
    curr = newlink;
  }

• 当您删除最后一个元素时，这就是它不起作用的原因

  Link  *curr;

  curr = head->next;   
  while (curr->next != NULL) {
    head = curr;
    curr = curr->next;
  }

  free(head->next);
  head->next = NULL;

• 当您想要删除列表的第一个元素时
  • 您不必释放字段值，因为您没有使用 malloc 分配它
  • 即使删除列表的第一个元素，也不会更改主列表开头的值。这就是为什么您必须将Link** 作为参数

void deleteFirst(Link **head){
  Link *curr;

  curr = (*head)->next;
  free(*head);
  *head = curr;
}

您可以通过提供列表开头的地址从主函数调用此函数：

  deleteFirst(&myList);
  deleteLast(myList);
  printList(myList);

当然，在所有函数中，您必须检查列表中是否至少有一些值，而不是空指针NULL

Answer 2

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

typedef struct node Node;
struct node{
    int element;
    Node *next;
};

Node* head = NULL;

bool put(int data)
{
    bool retVal = false;

    Node* temp;

    do
    {
        if (head == NULL)
        {
            head = malloc(sizeof(Node));

            if (head)
            {
                printf("New head created\n");
                head->element = data;
                head->next = NULL;
                printf("Element %d stored\n", data);

                retVal = true;
                break;
            }           
            else
            {           
                printf("Could not create new head\n");
                break;
            }   
        }

        temp = head;
        while (temp->next != NULL)
        {
            temp = temp->next;
        }

        temp->next = malloc(sizeof(Node));
        temp->next->element = data;
        temp->next->next = NULL;

        printf("Element %d stored\n", data);

        retVal = true;
        break;

    } while(1);

    return retVal;
}



bool get(int *pData)
{
    bool retVal = false;
    Node* temp;

    if (head != NULL)
    {
        retVal = true;
        Node *prevNode = head;

        temp = prevNode;
        while (temp->next)
        {
            prevNode = temp;
            temp = temp->next;
        }

        *pData = temp->element;
        printf("Element %d retrieved\n", *pData);

        free(temp);
        printf("Node freed\n");

        if (temp == head)
        {
            head = NULL;
        }
        else
        {
            prevNode->next = NULL;
        }   
    }

    return retVal;
}


int main(void)
{
    int retrievedNum;
    int num;

    for (num = 0; num < 5; num++)
    {
        put(num);
    }

    printf("\n");

    for (num = 0; num < 6; num++)
    {
        get(&retrievedNum);
    }

    put(num);

    get(&retrievedNum);

    printf("Element retrieved: %d\n", retrievedNum);

    return 1;
}

Answer 3

此代码将用于删除链接列表中的最后一个元素：

void dellast()

{

r=head;

struct node* z;
do
{
    z=r;
    r=r->next;
    if(r->next==NULL)
    {
        z->next=NULL;
        free(r->next);
    }   
}while(z->next!=NULL);
}

代码的工作方式如下：

跟踪当前节点及其前一个节点。

如果 current->next==NULL 表示它是最后一个节点。

previous->next=NULL 并释放当前节点也是如此

Answer 4

我用 C 语言制作了我自己的（单独的）LinkedList 示例，它被证明是有效的：

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

/********** GLOBALS *******************************/
#define OK 0
#define ERROR -1

/********** STRUCT AND TYPES DEFINTIONS ***********/
/* a node with key, data and reference to next node*/
typedef struct Node {
    int key;
    char string[1024];
    struct Node *next;  // pointer to next node
} Node;

/* the actual linked list: ref to first and last Node, size attribute */
typedef struct LinkedList {
    struct Node *first;
    struct Node *last;
    int size;
} LinkedList;

/********** FUNCTION HEADERS **********************/
LinkedList* init_list();
void insert_end(LinkedList *list, int key, char string[]);
void insert_beginning(LinkedList *list, int key, char string[]);
int remove_end(LinkedList *list);
int remove_beginning(LinkedList *list);
int print_list(LinkedList *list);
void free_list(LinkedList *list);
char * get_string(LinkedList *list, int key);

/*********** FUNCTION DEFINITIONS ***************/

/**
 * init_list Returns an appropriately (for an empty list) initialized struct List
 *
 * @return LinkedList *         ..ptr to the newly initialized list
 */
LinkedList * init_list() {
    printf("initializing list...\n");

    LinkedList *list = (LinkedList*) malloc(sizeof(LinkedList));

    list->first = NULL;
    list->last = NULL;
    list->size = 0;

    return list;
}

/**
 * Given a List, a key and a string adds a Node containing this
 * information at the end of the list
 *
 * @param list      LinkedList *    ..ptr to LinkedList
 * @param key       int             .. key of the Node to be inserted
 * @param string    char[]          .. the string of the Node to be inserted
 */
void insert_end(LinkedList *list, int key, char string[]) {
    printf("----------------------\n");

    list->size++;                    // increment size of list

    // intialize the new Node
    Node* newN = (Node*) malloc(sizeof(Node));
    newN->key = key;
    strcpy(newN->string, string);
    newN->next = NULL;

    Node* oldLast = list->last;      // get the old last
    oldLast->next = newN;          // make new Node the next Node for oldlast
    list->last = newN;              // set the new last  in the list

    printf("new Node(%p) at end: %d '%s' %p \n", newN, newN->key, newN->string,newN->next);
}

/**
 * Given a List, a key and a string adds a Node, containing
 * this information at the beginning of the list
 *
 * @param list      LinkedList *    ..ptr to LinkedList
 * @param key       int             .. key of the Node to be inserted
 * @param string    char[]          .. the string of the Node to be inserted
 */
void insert_beginning(LinkedList *list, int key, char string[]) {
    printf("----------------------\n");

    list->size++;                    // increment size of list
    Node* oldFirst = list->first;    //get the old first node

    /* intialize the new Node */
    Node* newN = (Node*) malloc(sizeof(Node));
    newN->key = key;
    strcpy(newN->string, string);
    newN->next = oldFirst;

    list->first = newN;              // set the new first

    /* special case: if list size == 1, then this new one is also the last one */
    if (list->size == 1)
        list->last = newN;

    printf("new Node(%p) at beginning: %d '%s' %p \n", newN, newN->key,newN->string, newN->next);
}

/**
 * Removes the first Node from the list
 *
 * @param list      LinkedList *        .. ptr to the List
 *
 * @return OK | ERROR
 */
int remove_beginning(LinkedList *list) {
    printf("----------------------\n");

    if (list->size <= 0)
        return ERROR;

    list->size--;

    Node * oldFirst = list->first;

    printf("delete Node(%p) at beginning: '%d' '%s' '%p' \n", oldFirst,oldFirst->key, oldFirst->string, oldFirst->next);

    free(list->first);          //free it
    list->first = oldFirst->next;
    oldFirst = NULL;

    return OK;
}

/**
 * Removes the last Node from the list.
 *
 * @param list      LinkedList *        .. ptr to the List
 *
 * @return OK | ERROR
 */
int remove_end(LinkedList *list) {
    printf("----------------------\n");

    /* special case #1 */
    if (list->size <= 0)
        return ERROR;

    /* special case #2 */
    if (list->size == 1) {
        free(list->first);
        list->first = NULL;
        list->last = NULL;
        return OK;
    }

    printf("delete Node(%p) at end: '%d' '%s' '%p' \n", list->last,list->last->key, list->last->string, list->last->next);

    list->size--;           // decrement list size
    Node * startNode = list->first;

    /* find the new last node (the one before the old last one); list->size >= 2 at this point!*/
    Node * newLast = startNode;
    while (newLast->next->next != NULL) {
        newLast = newLast->next;
    }

    free(newLast->next);    //free it
    newLast->next = NULL;   //set to NULL to denote new end of list
    list->last = newLast;   // set the new list->last

    return OK;
}

/**
 * Given a List prints all key/string pairs contained in the list to
 * the screen
 *
 * @param list      LinkedList *        .. ptr to the List
 *
 * @return  OK | ERROR
 */
int print_list(LinkedList *list) {

    printf("----------------------\n");

    if (list->size <= 0)
        return ERROR;

    printf("List.size = %d \n", list->size);

    Node *startN = list->first;  //get first

    /* iterate through list and print contents */
    do {
        printf("Node#%d.string = '%s', .next = '%p' \n", startN->key,startN->string, startN->next);
        startN = startN->next;
    } while (startN != NULL);

    return OK;
}

/**
 * Given a List, frees all memory associated with this list.
 *
 * @param list      LinkedList *        ..ptr to the list
 */
void free_list(LinkedList *list) {
    printf("----------------------\n");
    printf("freeing list...\n");

    if (list != NULL && list->size > 0) {
        Node * startN = list->first;
        Node * temp = list->first;

        do {
            free(temp);
            startN = startN->next;
            temp = startN;
        } while (startN != NULL);

        free(list);
    }
}

/**
 * Given a List and a key, iterates through the whole List and returns
 * the string of the first node which contains the key
 *
 * @param list      LinkedList *        ..ptr to the list
 * @param key       int                 .. the key of the Node to get the String from
 *
 * @return OK | ERROR
 */
char * get_string(LinkedList *list, int key) {
    printf("----------------------\n");

    Node *startN = list->first;  //get first

    /* iterate through list and find Node where node->key == key */
    while (startN->next != NULL) {
        if (startN->key == key)
            return startN->string;
        else
            startN = startN->next;
    }

    return NULL;
}

/*************** MAIN **************/
int main(void) {

    LinkedList *list = init_list();

    insert_beginning(list, 1, "im the first");
    insert_end(list, 2, "im the second");
    insert_end(list, 3, "im the third");
    insert_end(list, 4, "forth here");

    print_list(list);
    remove_end(list);
    print_list(list);
    remove_beginning(list);
    print_list(list);
    remove_end(list);
    print_list(list);
    printf("string at node with key %d = '%s' \n",2,get_string(list, 2));
    free_list(list);

    return OK;
}

希望这个实现示例有所帮助。