我尝试在单链表上执行插入和删除操作，但deletelast()（删除链表的最后一个节点）函数不适用于具有2个以上节点的链表。
下面是代码：

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

struct node {
    int data;
    struct node *next;
};
typedef struct node node;

node *head = NULL;

void insertbegin(int *a)
{
    node *newnode = (node *)malloc(sizeof(node));
    newnode->next = head;
    head = newnode;
    newnode->data = *a;
    printf("success!!");
}

void display()
{
    if (head != NULL)
    {
        node *temp = head;
        while (temp != NULL)
        {
            printf("%d\t", temp->data);
            temp = temp->next;
        }
    }
    else if (head == NULL)
    {
        printf("the linked list is empty\n");
    }
}

void insertlast()
{
    int b;
    printf("enter the value of the element:\n");
    scanf("%d", &b);
    node *temp = head;
    node *newnode = (node *)malloc(sizeof(node));
    while (temp != NULL)
    {
        if (temp->next == NULL)
        {
            temp->next = newnode;
            newnode->next = NULL;
            newnode->data = b;
        }
        temp = temp->next;
    }
    free(temp);
    temp = NULL;
}

void insertspecific()
{
    int n, b;
    printf("enter the value after which you want to insert a node: \n");
    scanf("%d", &n);
    printf("enter te data of the node to be inserted: \n");
    scanf("%d", &b);
    node *temp = head;
    node *newnode = (node *)malloc(sizeof(node));
    while (temp != NULL)
    {
        if (temp->data == n)
        {
            newnode->next = temp->next;
            temp->next = newnode;
            newnode->data = b;
        }
        temp = temp->next;
    }
    free(temp);
}

void deletebegin()
{
    if (head == NULL)
        printf("the list is empty.\n");
    else
    {
        node *newhead = head;
        head = newhead->next;
        printf("success deletion");
        free(newhead);
    }
}

void deletelast()
{
    if (head == NULL)
    {
        printf("the list is empty\n");
    }
    else if (head->next->next == NULL)
    {
        node *temp = NULL;
        temp = head->next;
        head->next = NULL;
        free(temp);
        temp = NULL;
    }
    else
    {
        node *temp = head;
        node *ptr = NULL;
        while (temp != NULL)
        {
            if (temp->next->next == NULL)
            {  
                ptr = temp->next;
                temp->next == NULL;
                free(ptr);
                ptr = NULL;
            }
            temp = temp->next;
        }
        printf("deleted the node from the last\n");
    }
}

void main()
{
    int a;
    int choice = 0;
    while (choice != 9)
    {
        printf("\n\n*********Main Menu*********\n");
        printf("\nChoose one option from the following list ...\n");
        printf("\n===============================================\n");
        printf("\n1.Insert at begining\n2.Insert at last\n3.Insert after a specific position\n4.Delete from Beginning\n5.Delete from last\n6.Delete node after specified location\n7.traverse\n8.Display the Linked list\n9.Exit\n");
        printf("\nEnter your choice?\n");
        scanf("\n%d", &choice);
        switch (choice)
        {
        case 1:
            printf("enter the value you want to insert: \n");
            scanf("%d", &a);
            insertbegin(&a);
            break;
        case 2:
            insertlast();
            break;
        case 3:
            insertspecific();
            break;
        case 4:
            deletebegin();
            break;
        case 5:
            deletelast();
            break;
        case 6:
            // deletespecific();
            break;
        case 7:
            // traverse();
            break;
        case 8:
            display();
            break;
        case 9:
            exit(0);
            break;
        default:
            printf("Please enter valid choice..");
        }
    }
}

我正在使用insertbegin()函数插入节点，并使用display()函数查看链表，但deletelast()函数正在工作，并且没有为超过2个节点提供输出。我的逻辑是使用temp指针遍历链表，当我到达倒数第二个节点时，我将其next值更改为NULL，并使用ptr指针释放最后一个节点。但由于某种原因，它不起作用。
我对DSA还是个新手。
运行deletelast()函数时的输出屏幕