C 中删除列表节点的这一部分是什么意思

Question

下面是删除链接的简单列表的最后一个节点的完整代码，如果它运行良好，函数是void deleteLastNode()。

但这部分到底是什么意思？在我看来，这是多余的，因为当 while 循环结束时，我们的 toDelete 节点位于最后一个位置，而 secondLastNode 位于倒数第二个。

if(toDelete == head)
        {
            head = NULL;
        }

deleteLastNode () 函数是这样的：

void deleteLastNode()
{
    struct node *toDelete, *secondLastNode;

    if(head == NULL)
    {
        printf("List is already empty.");
    }
    else
    {
        toDelete = head;
        secondLastNode = head;

        /* Traverse to the last node of the list */
        while(toDelete->next != NULL)
        {
            secondLastNode = toDelete;
            toDelete = toDelete->next;
        }

        if(toDelete == head)
        {
            head = NULL;
        }
        else
        {
            /* Disconnect link of second last node with last node */
            secondLastNode->next = NULL;
        }

        /* Delete the last node */
        free(toDelete);

        printf("SUCCESSFULLY DELETED LAST NODE OF LIST\n");
    }
}

但我相信这已经足够了：

/*  */
void deleteLastNode()
{
    struct node *toDelete, *secondLastNode;

    if(head == NULL)
    {
        printf("List is already empty.");
    }
    else
    {
        toDelete = head;
        secondLastNode = head;

        /* Traverse to the last node of the list */
        while(toDelete->next != NULL)
        {
            secondLastNode = toDelete;
            toDelete = toDelete->next;
        }





            /* Disconnect link of second last node with last node */
            secondLastNode->next = NULL;


        /* Delete the last node */
        free(toDelete);

        printf("SUCCESSFULLY DELETED LAST NODE OF LIST\n");
    }
}

完整的代码是这样的：函数是void deleteLastNode()::

/**
 * C program to delete last node of Singly Linked List
 */

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


/* Structure of a node */
struct node {
    int data;          // Data
    struct node *next; // Address 
}*head;

void createList(int n);
void deleteLastNode();
void displayList();


int main()
{
    int n, choice;

    /*
     * Create a singly linked list of n nodes
     */
    printf("Enter the total number of nodes: ");
    scanf("%d", &n);
    createList(n);

    printf("\nData in the list \n");
    displayList();

    printf("\nPress 1 to delete last node: ");
    scanf("%d", &choice);

    /* Delete last node from list */
    if(choice == 1)
        deleteLastNode();

    printf("\nData in the list \n");
    displayList();

    return 0;
}


/*
 * Create a list of n nodes
 */
void createList(int n)
{
    struct node *newNode, *temp;
    int data, i;

    head = (struct node *)malloc(sizeof(struct node));

    /*
     * If unable to allocate memory for head node
     */
    if(head == NULL)
    {
        printf("Unable to allocate memory.");
    }
    else
    {
        /*
         * Input data of node from the user
         */
        printf("Enter the data of node 1: ");
        scanf("%d", &data);

        head->data = data; // Link the data field with data
        head->next = NULL; // Link the address field to NULL

        temp = head;

        /*
         * Create n nodes and adds to linked list
         */
        for(i=2; i<=n; i++)
        {
            newNode = (struct node *)malloc(sizeof(struct node));

            /* If memory is not allocated for newNode */
            if(newNode == NULL)
            {
                printf("Unable to allocate memory.");
                break;
            }
            else
            {
                printf("Enter the data of node %d: ", i);
                scanf("%d", &data);

                newNode->data = data; // Link the data field of newNode with data
                newNode->next = NULL; // Link the address field of newNode with NULL

                temp->next = newNode; // Link previous node i.e. temp to the newNode
                temp = temp->next;
            }
        }

        printf("SINGLY LINKED LIST CREATED SUCCESSFULLY\n");
    }
}


/*
 * Delete last node of the linked list
 */

void deleteLastNode()
{
    struct node *toDelete, *secondLastNode;

    if(head == NULL)
    {
        printf("List is already empty.");
    }
    else
    {
        toDelete = head;
        secondLastNode = head;

        /* Traverse to the last node of the list */
        while(toDelete->next != NULL)
        {
            secondLastNode = toDelete;
            toDelete = toDelete->next;
        }

        if(toDelete == head)
        {
            head = NULL;
        }
        else
        {
            /* Disconnect link of second last node with last node */
            secondLastNode->next = NULL;
        }

        /* Delete the last node */
        free(toDelete);

        printf("SUCCESSFULLY DELETED LAST NODE OF LIST\n");
    }
}


/*
 * Display entire list
 */
void displayList()
{
    struct node *temp;

    /*
     * If the list is empty i.e. head = NULL
     */
    if(head == NULL)
    {
        printf("List is empty.");
    }
    else
    {
        temp = head;
        while(temp != NULL)
        {
            printf("Data = %d\n", temp->data); // Print the data of current node
            temp = temp->next;                // Move to next node
        }
    }
}

谢谢

Answer 1

在第二个函数实现中，如果最后一个节点是头节点，那么变量头本身不会改变。因此，当列表仅包含一个节点即头节点时，该函数可能具有未定义的行为。

请注意，无论如何功能实现都是不好的。例如，该函数处理全局变量 head。所以在一个程序中你不能定义两个列表。

函数可以写得更简单。

void deleteLastNode( struct node **head )
{
    if ( *head != NULL )
    {
        while ( ( *head )->next != NULL ) head = &( *head )->next;

        free( *head );

        *head = NULL;
    }
}

并像这样称呼

deleteLastNode( &head );