刪除雙鏈表開頭節點

刪除雙向鏈表中的第一個節點是最簡單的操作。只需要將頭指針複製到指針ptr並將頭指針移動到下一個指針。

ptr = head;
head = head -> next;

現在使這個新頭節點的prev指針指向NULL。這將通過使用以下語句來完成。

head -> prev = NULL;

現在使用free函數釋放指針ptr。

free(ptr);

演算法

第1步：如果HEAD = NULL
    提示 記憶體溢出。

    轉到第6步

第2步：設置PTR = HEAD
第3步：設置HEAD = HEAD->NEXT
第4步：設置HEAD->PREV = NULL
第5步：釋放PTR
第6步：退出

示意圖

C語言實現示例代碼 -

#include<stdio.h>
#include<stdlib.h>
void create(int);
void beginning_delete();
struct node
{
    int data;
    struct node *next;
    struct node *prev;
};
struct node *head;
void main()
{
    int choice, item;
    do
    {
        printf("1.Append List\n");
        printf("2.Delete node from beginning\n");
        printf("3.Exit\n");
        printf("4.Enter your choice ? ");
        scanf("%d", &choice);
        switch (choice)
        {
        case 1:
            printf("Enter the item\n");
            scanf("%d", &item);
            create(item);
            break;
        case 2:
            beginning_delete();
            break;
        case 3:
            exit(0);
            break;
        default:
            printf("Please enter valid choice\n");
        }

    } while (choice != 3);
}
void create(int item)
{

    struct node *ptr = (struct node *)malloc(sizeof(struct node));
    if (ptr == NULL)
    {
        printf("OVERFLOW\n");
    }
    else
    {
        if (head == NULL)
        {
            ptr->next = NULL;
            ptr->prev = NULL;
            ptr->data = item;
            head = ptr;
        }
        else
        {
            ptr->data = item;printf("Press 0 to insert more ?\n");
            ptr->prev = NULL;
            ptr->next = head;
            head->prev = ptr;
            head = ptr;
        }
        printf("Node Inserted\n");
    }

}
void beginning_delete()
{
    struct node *ptr;
    if (head == NULL)
    {
        printf("UNDERFLOW\n");
    }
    else if (head->next == NULL)
    {
        head = NULL;
        free(head);
        printf("Node Deleted\n");
    }
    else
    {
        ptr = head;
        head = head->next;
        head->prev = NULL;
        free(ptr);
        printf("Node Deleted\n");
    }
}

執行上面示例代碼，得到以下結果 -

1.Append List
2.Delete node from beginning
3.Exit
4.Enter your choice?1

Enter the item
12

Node Inserted
1.Append List
2.Delete node from beginning
3.Exit
4.Enter your choice?2

Node Deleted
1.Append List
2.Delete node from beginning
3.Exit
4.Enter your choice?