数据结构-循环链表

循环链表是链表的一种变体，其中第一个元素指向最后一个元素，最后一个元素指向第一个元素。单链表和双向链表都可以做成循环链表。

作为循环的单向链表

在单链表中，最后一个节点的next指针指向第一个节点。

循环双向链表

在双向链表中，最后一个节点的next指针指向第一个节点，第一个节点的previous指针指向最后一个节点，双向循环。

根据上图，以下是需要考虑的要点。

• 无论是单链表还是双链表，最后一个链接的 next 都指向列表的第一个链接。

• 在双向链表的情况下，第一个链接的前一个指向列表的最后一个。

基本操作

以下是循环列表支持的重要操作。

• insert - 在列表的开头插入一个元素。

• delete - 从列表开头删除一个元素。

• 显示- 显示列表。

插入操作

循环链表的插入操作仅将元素插入链表的开头。这与通常的单链表和双向链表不同，因为该列表中没有特定的起点和终点。插入是在列表中特定节点（或给定位置）的开头或之后完成的。

算法

1. START
2. Check if the list is empty
3. If the list is empty, add the node and point the head to this node
4. If the list is not empty, link the existing head as the next node to the new node.
5. Make the new node as the new head.
6. END

例子

以下是此操作在各种编程语言中的实现 -

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>
struct node {
   int data;
   int key;
   struct node *next;
};
struct node *head = NULL;
struct node *current = NULL;
bool isEmpty(){
   return head == NULL;
}

//insert link at the first location
void insertFirst(int key, int data){

   //create a link
   struct node *link = (struct node*) malloc(sizeof(struct node));
   link->key = key;
   link->data = data;
   if (isEmpty()) {
      head = link;
      head->next = head;
   } else {

      //point it to old first node
      link->next = head;

      //point first to new first node
      head = link;
   }
}

//display the list
void printList(){
   struct node *ptr = head;
   printf("\n[ ");

   //start from the beginning
   if(head != NULL) {
      while(ptr->next != ptr) {
         printf("(%d,%d) ",ptr->key,ptr->data);
         ptr = ptr->next;
      }
   }
   printf(" ]");
}
void main(){
   insertFirst(1,10);
   insertFirst(2,20);
   insertFirst(3,30);
   insertFirst(4,1);
   insertFirst(5,40);
   insertFirst(6,56);
   printf("Circular Linked List: ");

   //print list
   printList();
}

Circular Linked List: 
[ (6,56) (5,40) (4,1) (3,30) (2,20)  ]

#include <iostream>
#include <cstring>
#include <cstdlib>
#include <cstdbool>
struct node {
   int data;
   int key;
   struct node *next;
};
struct node *head = NULL;
struct node *current = NULL;
bool isEmpty(){
   return head == NULL;
}
//insert link at the first location
void insertFirst(int key, int data){

   //create a link
   struct node *link = (struct node*) malloc(sizeof(struct node));
   link->key = key;
   link->data = data;
   if (isEmpty()) {
      head = link;
      head->next = head;
   } else {

      //point it to old first node
      link->next = head;

      //point first to new first node
      head = link;
   }
}

//display the list
void printList(){
   struct node *ptr = head;
   printf("\n[ ");

   //start from the beginning
   if(head != NULL) {
      while(ptr->next != ptr) {
         printf("(%d,%d) ",ptr->key,ptr->data);
         ptr = ptr->next;
      }
   }
   printf(" ]");
}
int main(){
   insertFirst(1,10);
   insertFirst(2,20);
   insertFirst(3,30);
   insertFirst(4,1);
   insertFirst(5,40);
   insertFirst(6,56);
   printf("Circular Linked List: ");
   
   //print list
   printList();
   return 0;
}

Circular Linked List: 
[ (6,56) (5,40) (4,1) (3,30) (2,20)  ]

//Java program for circular link list
import java.util.*;
class Node {
    int data;
    int key;
    Node next;
}
public class Main {
    static Node head = null;
    static Node current = null;
    static boolean isEmpty() {
        return head == null;
    }
    //insert link at the first location
    static void insertFirst(int key, int data) {
        //create a link
        Node link = new Node();
        link.key = key;
        link.data = data;
        if (isEmpty()) {
            head = link;
            head.next = head;
        } else {
            //point it to old first node
            link.next = head;
            //point first to new first node
            head = link;
        }
    }
    //display the list
    static void printList() {
        Node ptr = head;
        System.out.print("\n[ ");
        //start from the beginning
        if (head != null) {
            while (ptr.next != ptr) {
                System.out.print("(" + ptr.key + "," + ptr.data + ") ");
                ptr = ptr.next;
            }
        }
        System.out.print(" ]");
    }
    public static void main(String[] args) {
        insertFirst(1, 10);
        insertFirst(2, 20);
        insertFirst(3, 30);
        insertFirst(4, 1);
        insertFirst(5, 40);
        insertFirst(6, 56);
        System.out.print("Circular Linked List: ");
        //print list
        printList();
    }
}

Circular Linked List: [ (6,56) (5,40) (4,1) (3,30) (2,20) ]

#python program for circular linked list
class Node:
    def __init__(self, key, data):
        self.key = key
        self.data = data
        self.next = None
head = None
current = None
def is_empty():
    return head is None
#insert link at the first location
def insert_first(key, data):
    #create a link
    global head
    new_node = Node(key, data)
    if is_empty():
        head = new_node
        head.next = head
    else:
        #point it to old first node
        new_node.next = head
        #point first to the new first node
        head = new_node
        #display the list
def print_list():
    global head
    ptr = head
    print("[", end=" ")
    #start from the beginning
    if head is not None:
        while ptr.next != ptr:
            print("({}, {})".format(ptr.key, ptr.data), end=" ")
            ptr = ptr.next
        print("]")
insert_first(1, 10)
insert_first(2, 20)
insert_first(3, 30)
insert_first(4, 1)
insert_first(5, 40)
insert_first(6, 56)
#printlist
print("Circular Linked List: ")
print_list()

Circular Linked List: [ (6,56) (5,40) (4,1) (3,30) (2,20) ]

删除操作

循环链表中的删除操作从链表中删除某个节点。此类列表中的删除操作可以在开头、给定位置或结尾处进行。

算法

1. START
2. If the list is empty, then the program is returned.
3. If the list is not empty, we traverse the list using a current pointer that is set to the head pointer and create another pointer previous that points to the last node.
4. Suppose the list has only one node, the node is deleted by setting the head pointer to NULL.
5. If the list has more than one node and the first node is to be deleted, the head is set to the next node and the previous is linked to the new head.
6. If the node to be deleted is the last node, link the preceding node of the last node to head node.
7. If the node is neither first nor last, remove the node by linking its preceding node to its succeeding node.
8. END

例子

以下是此操作在各种编程语言中的实现 -

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>
struct node {
   int data;
   int key;
   struct node *next;
};
struct node *head = NULL;
struct node *current = NULL;
bool isEmpty(){
   return head == NULL;
}

//insert link at the first location
void insertFirst(int key, int data){

   //create a link
   struct node *link = (struct node*) malloc(sizeof(struct node));
   link->key = key;
   link->data = data;
   if (isEmpty()) {
      head = link;
      head->next = head;
   } else {

      //point it to old first node
      link->next = head;

      //point first to new first node
      head = link;
   }
}

//delete first item
struct node * deleteFirst(){

   //save reference to first link
   struct node *tempLink = head;
   if(head->next == head) {
      head = NULL;
      return tempLink;
   }

   //mark next to first link as first
   head = head->next;

   //return the deleted link
   return tempLink;
}

//display the list
void printList(){
   struct node *ptr = head;

   //start from the beginning
   if(head != NULL) {
      while(ptr->next != ptr) {
         printf("(%d,%d) ",ptr->key,ptr->data);
         ptr = ptr->next;
      }
   }
}
void main(){
   insertFirst(1,10);
   insertFirst(2,20);
   insertFirst(3,30);
   insertFirst(4,1);
   insertFirst(5,40);
   insertFirst(6,56);
   printf("Circular Linked List: ");

   //print list
   printList();
   deleteFirst();
   printf("\nList after deleting the first item: ");
   printList();
}

Circular Linked List: (6,56) (5,40) (4,1) (3,30) (2,20) 
List after deleting the first item: (5,40) (4,1) (3,30) (2,20) 

#include <iostream>
#include <cstring>
#include <cstdlib>
#include <cstdbool>
struct node {
   int data;
   int key;
   struct node *next;
};
struct node *head = NULL;
struct node *current = NULL;
bool isEmpty(){
   return head == NULL;
}

//insert link at the first location
void insertFirst(int key, int data){

   //create a link
   struct node *link = (struct node*) malloc(sizeof(struct node));
   link->key = key;
   link->data = data;
   if (isEmpty()) {
      head = link;
      head->next = head;
   } else {

      //point it to old first node
      link->next = head;

      //point first to new first node
      head = link;
   }
}

//delete first item
struct node * deleteFirst(){
   
   //save reference to first link
   struct node *tempLink = head;
   if(head->next == head) {
      head = NULL;
      return tempLink;
   }

   //mark next to first link as first
   head = head->next;

   //return the deleted link
   return tempLink;
}

//display the list
void printList(){
   struct node *ptr = head;

   //start from the beginning
   if(head != NULL) {
      while(ptr->next != ptr) {
         printf("(%d,%d) ",ptr->key,ptr->data);
         ptr = ptr->next;
      }
   }
}
int main(){
   insertFirst(1,10);
   insertFirst(2,20);
   insertFirst(3,30);
   insertFirst(4,1);
   insertFirst(5,40);
   insertFirst(6,56);
   printf("Circular Linked List: ");

   //print list
   printList();
   deleteFirst();
   printf("\nList after deleting the first item: ");
   printList();
   return 0;
}

Circular Linked List: (6,56) (5,40) (4,1) (3,30) (2,20) 
List after deleting the first item: (5,40) (4,1) (3,30) (2,20) 

//Java program for circular linked list
import java.util.*;
public class Main {
    static class Node {
        int data;
        int key;
        Node next;
    }
    static Node head = null;
    static Node current = null;

    static boolean isEmpty() {
        return head == null;
    }
    //insert link at the first location
    static void insertFirst(int key, int data) {
        //create a link
        Node link = new Node();
        link.key = key;
        link.data = data;
        if (isEmpty()) {
            head = link;
            head.next = head;
        } else {
            //point it to old first node
            link.next = head;
            //point first to new first node
            head = link;
        }
    }
//delete first item
    static Node deleteFirst() {
        //save reference to first link
        Node tempLink = head;
        if (head.next == head) {
            head = null;
            return tempLink;
        }
        //mark next to first link as first
        head = head.next;
        //return the deleted link
        return tempLink;
    }
    //display the list
    static void printList() {
        Node ptr = head;
         //start from the beginning
        if (head != null) {
            while (ptr.next != ptr) {
                System.out.printf("(%d,%d) ", ptr.key, ptr.data);
                ptr = ptr.next;
            }
        }
    }
    public static void main(String[] args) {
        insertFirst(1, 10);
        insertFirst(2, 20);
        insertFirst(3, 30);
        insertFirst(4, 1);
        insertFirst(5, 40);
        insertFirst(6, 56);
        System.out.print("Circular Linked List: ");
        //print list
        printList();
        deleteFirst();
  }
}

Circular Linked List: (6,56) (5,40) (4,1) (3,30) (2,20) 
List after deleting the first item: (5,40) (4,1) (3,30) (2,20)

#python program for circular linked list
class Node:
    def __init__(self, key, data):
        self.key = key
        self.data = data
        self.next = None
head = None
current = None
def is_empty():
    return head is None
#insert link at the first location
def insert_first(key, data):
    #create a link
    global head
    new_node = Node(key, data)
    if is_empty():
        head = new_node
        head.next = head
    else:
        #point it to old first node
        new_node.next = head
        #point first to the new first node
        head = new_node
        
def print_list():
    global head
    ptr = head
    print("[", end=" ")
    #start from the beginning
    if head is not None:
        while ptr.next != ptr:
            print("({}, {})".format(ptr.key, ptr.data), end=" ")
            ptr = ptr.next
        print("]")
def delete_first():
    global head
    temp_link = head
    if head.next == head:
        head = None
        return temp_link
    head = head.next
    return temp_link  
insert_first(1, 10)
insert_first(2, 20)
insert_first(3, 30)
insert_first(4, 1)
insert_first(5, 40)
insert_first(6, 56)
#printlist
print("Circular Linked List: ")
print_list()
delete_first()
print("\nList after deleting the first item: ")
print_list();

Circular Linked List: (6,56) (5,40) (4,1) (3,30) (2,20) 
List after deleting the first item: (5,40) (4,1) (3,30) (2,20)

显示列表操作

显示列表操作访问列表中的每个节点并将它们全部打印在输出中。

算法

1. START
2. Walk through all the nodes of the list and print them
3. END

例子

以下是此操作在各种编程语言中的实现 -

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>
struct node {
   int data;
   int key;
   struct node *next;
};
struct node *head = NULL;
struct node *current = NULL;
bool isEmpty(){
   return head == NULL;
}

//insert link at the first location
void insertFirst(int key, int data){

   //create a link
   struct node *link = (struct node*) malloc(sizeof(struct node));
   link->key = key;
   link->data = data;
   if (isEmpty()) {
      head = link;
      head->next = head;
   } else {

      //point it to old first node
      link->next = head;

      //point first to new first node
      head = link;
   }
}

//display the list
void printList(){
   struct node *ptr = head;
   printf("\n[ ");

   //start from the beginning
   if(head != NULL) {
      while(ptr->next != ptr) {
         printf("(%d,%d) ",ptr->key,ptr->data);
         ptr = ptr->next;
      }
   }
   printf(" ]");
}
void main(){
   insertFirst(1,10);
   insertFirst(2,20);
   insertFirst(3,30);
   insertFirst(4,1);
   insertFirst(5,40);
   insertFirst(6,56);
   printf("Circular Linked List: ");

   //print list
   printList();
}

Circular Linked List: 
[ (6,56) (5,40) (4,1) (3,30) (2,20)  ]

#include <iostream>
#include <cstring>
#include <cstdlib>
#include <cstdbool>
struct node {
   int data;
   int key;
   struct node *next;
};
struct node *head = NULL;
struct node *current = NULL;
bool isEmpty(){
   return head == NULL;
}

//insert link at the first location
void insertFirst(int key, int data){
   
   //create a link
   struct node *link = (struct node*) malloc(sizeof(struct node));
   link->key = key;
   link->data = data;
   if (isEmpty()) {
      head = link;
      head->next = head;
   } else {

      //point it to old first node
      link->next = head;

      //point first to new first node
      head = link;
   }
}

//display the list
void printList(){
   struct node *ptr = head;
   printf("\n[ ");

   //start from the beginning
   if(head != NULL) {
      while(ptr->next != ptr) {
         printf("(%d,%d) ",ptr->key,ptr->data);
         ptr = ptr->next;
      }
   }
   printf(" ]");
}
int main(){
   insertFirst(1,10);
   insertFirst(2,20);
   insertFirst(3,30);
   insertFirst(4,1);
   insertFirst(5,40);
   insertFirst(6,56);
   printf("Circular Linked List: ");

   //print list
   printList();
   return 0;
}

Circular Linked List: 
[ (6,56) (5,40) (4,1) (3,30) (2,20)  ]

//Java program for circular link list
import java.util.*;
class Node {
    int data;
    int key;
    Node next;
}
public class Main {
    static Node head = null;
    static Node current = null;
    static boolean isEmpty() {
        return head == null;
    }
    //insert link at the first location
    static void insertFirst(int key, int data) {
        //create a link
        Node link = new Node();
        link.key = key;
        link.data = data;
        if (isEmpty()) {
            head = link;
            head.next = head;
        } else {
            //point it to old first node
            link.next = head;
            //point first to new first node
            head = link;
        }
    }
    //display the list
    static void printList() {
        Node ptr = head;
        System.out.print("\n[ ");
        //start from the beginning
        if (head != null) {
            while (ptr.next != ptr) {
                System.out.print("(" + ptr.key + "," + ptr.data + ") ");
                ptr = ptr.next;
            }
        }
        System.out.print(" ]");
    }
    public static void main(String[] args) {
        insertFirst(1, 10);
        insertFirst(2, 20);
        insertFirst(3, 30);
        insertFirst(4, 1);
        insertFirst(5, 40);
        insertFirst(6, 56);
        System.out.print("Circular Linked List: ");
        //print list
        printList();
    }
}

Circular Linked List: [ (6,56) (5,40) (4,1) (3,30) (2,20) ]

#python program for circular linked list
class Node:
    def __init__(self, key, data):
        self.key = key
        self.data = data
        self.next = None
head = None
current = None
def is_empty():
    return head is None
#insert link at the first location
def insert_first(key, data):
    #create a link
    global head
    new_node = Node(key, data)
    if is_empty():
        head = new_node
        head.next = head
    else:
        #point it to old first node
        new_node.next = head
        #point first to the new first node
        head = new_node
        #display the list
def print_list():
    global head
    ptr = head
    print("[", end=" ")
    #start from the beginning
    if head is not None:
        while ptr.next != ptr:
            print("({}, {})".format(ptr.key, ptr.data), end=" ")
            ptr = ptr.next
        print("]")
insert_first(1, 10)
insert_first(2, 20)
insert_first(3, 30)
insert_first(4, 1)
insert_first(5, 40)
insert_first(6, 56)
#printlist
print("Circular Linked List: ")
print_list()

Circular Linked List: [ (6,56) (5,40) (4,1) (3,30) (2,20) ]

完成实施

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>
struct node {
   int data;
   int key;
   struct node *next;
};
struct node *head = NULL;
struct node *current = NULL;
bool isEmpty(){
   return head == NULL;
}
int length(){
   int length = 0;
   //if list is empty
   if(head == NULL) {
      return 0;
   }
   current = head->next;
   while(current != head) {
      length++;
      current = current->next;
   }
   return length;
}
//insert link at the first location
void insertFirst(int key, int data){
   //create a link
   struct node *link = (struct node*) malloc(sizeof(struct node));
   link->key = key;
   link->data = data;
   if (isEmpty()) {
      head = link;
      head->next = head;
   } else {
      //point it to old first node
      link->next = head;

      //point first to new first node
      head = link;
   }
}
//delete first item
struct node * deleteFirst(){

   //save reference to first link
   struct node *tempLink = head;
   if(head->next == head) {
      head = NULL;
      return tempLink;
   }
   //mark next to first link as first
   head = head->next;

   //return the deleted link
   return tempLink;
}
//display the list
void printList(){
   struct node *ptr = head;
   printf("\n[ ");
   //start from the beginning
   if(head != NULL) {
      while(ptr->next != ptr) {
         printf("(%d,%d) ",ptr->key,ptr->data);
         ptr = ptr->next;
      }
   }
   printf(" ]");
}
int main(){
   insertFirst(1,10);
   insertFirst(2,20);
   insertFirst(3,30);
   insertFirst(4,1);
   insertFirst(5,40);
   insertFirst(6,56);
   printf("Original List: ");
   //print list
   printList();
   while(!isEmpty()) {
      struct node *temp = deleteFirst();
      printf("\nDeleted value:");
      printf("(%d,%d) ",temp->key,temp->data);
   }
   printf("\nList after deleting all items: ");
   printList();
}

Original List: 
[ (6,56) (5,40) (4,1) (3,30) (2,20)  ]
Deleted value:(6,56) 
Deleted value:(5,40) 
Deleted value:(4,1) 
Deleted value:(3,30) 
Deleted value:(2,20) 
Deleted value:(1,10) 
List after deleting all items: 
[  ]

#include <iostream>
#include <cstring>
#include <cstdlib>
#include <cstdbool>
using namespace std;
struct node {
   int data;
   int key;
   struct node *next;
};
struct node *head = NULL;
struct node *current = NULL;
bool isEmpty(){
   return head == NULL;
}
int length(){
   int length = 0;
   
   //if list is empty
   if(head == NULL) {
      return 0;
   }
   current = head->next;
   while(current != head) {
      length++;
      current = current->next;
   }
   return length;
}
//insert link at the first location
void insertFirst(int key, int data){
   //create a link
   struct node *link = (struct node*) malloc(sizeof(struct node));
   link->key = key;
   link->data = data;
   if (isEmpty()) {
      head = link;
      head->next = head;
   } else {
      //point it to old first node
      link->next = head;
      
      //point first to new first node
      head = link;
   }
}
//delete first item
struct node * deleteFirst(){   
   //save reference to first link
   struct node *tempLink = head;
   if(head->next == head) {
      head = NULL;
      return tempLink;
   }
   //mark next to first link as first
   head = head->next;
   
   //return the deleted link
   return tempLink;
}
//display the list
void printList(){
   struct node *ptr = head;
   cout << "\n[ ";
   //start from the beginning
   if(head != NULL) {
      while(ptr->next != ptr) {
         cout << "(" << ptr->key << "," << ptr->data << ") ";
         ptr = ptr->next;
      }
   }
   cout << " ]";
}
int main(){
   insertFirst(1,10);
   insertFirst(2,20);
   insertFirst(3,30);
   insertFirst(4,1);
   insertFirst(5,40);
   insertFirst(6,56);
   cout << "Original List: ";
   //print list
   printList();
   while(!isEmpty()) {
      struct node *temp = deleteFirst();
      cout << "\n Deleted value:";
      cout << "(" << temp->key << "," << temp->data << ") ";
   }
   cout << "\n List after deleting all items: ";
   printList();
   return 0;
}

Original List: 
[ (6,56) (5,40) (4,1) (3,30) (2,20)  ]
 Deleted value:(6,56) 
 Deleted value:(5,40) 
 Deleted value:(4,1) 
 Deleted value:(3,30) 
 Deleted value:(2,20) 
 Deleted value:(1,10) 
 List after deleting all items: 
[  ]

class Node {
    int data;
    int key;
    Node next;
    Node(int key, int data) {
        this.key = key;
        this.data = data;
        this.next = null;
    }
}
public class LinkedList {
    private Node head;
    private Node current;
    boolean isEmpty() {
        return head == null;
    }
    int length() {
        int length = 0;
        //if list is empty
        if (head == null) {
            return 0;
        }
        current = head.next;
        while (current != head) {
            length++;
            current = current.next;
        }
        return length;
    }
    //insert link at the first location
    void insertFirst(int key, int data) {
        //create a link
        Node link = new Node(key, data);
        if (isEmpty()) {
            head = link;
            head.next = head;
        } else {
            //point it to old first node
            link.next = head;
            //point first to new first node
            head = link;
        }
    }
    //delete first item
    Node deleteFirst() {
        if (head.next == head) {
            //save reference to first link
            Node tempLink = head;
            head = null;
            return tempLink;
        }
        Node tempLink = head;
        //mark next to first link as first
        head = head.next;
        //return the deleted link
        return tempLink;
    }
    //display the list
    void printList() {
        Node ptr = head;
        System.out.print("\n[ ");

        //start from the beginning
        if (head != null) {
            while (ptr.next != ptr) {
                System.out.print("(" + ptr.key + "," + ptr.data + ") ");
                ptr = ptr.next;
            }
        }
        System.out.print(" ]");
    }
    public static void main(String[] args) {
        LinkedList linkedList = new LinkedList();
        linkedList.insertFirst(1, 10);
        linkedList.insertFirst(2, 20);
        linkedList.insertFirst(3, 30);
        linkedList.insertFirst(4, 1);
        linkedList.insertFirst(5, 40);
        linkedList.insertFirst(6, 56);
        System.out.print("Original List: ");
        linkedList.printList();
        //print list
        while (!linkedList.isEmpty()) {
            Node temp = linkedList.deleteFirst();
            System.out.println("\nDeleted value: (" + temp.key + "," + temp.data + ")");
        }
        System.out.print("\nList after deleting all items: ");
        linkedList.printList();
    }
}

Original List: 
[ (6,56) (5,40) (4,1) (3,30) (2,20)  ]
Deleted value: (6,56)

Deleted value: (5,40)

Deleted value: (4,1)

Deleted value: (3,30)

Deleted value: (2,20)

Deleted value: (1,10)

List after deleting all items: 
[  ]

class Node:
    def __init__(self, key, data):
        self.key = key
        self.data = data
        self.next = None
class LinkedList:
    def __init__(self):
        self.head = None
        self.current = None
    def is_empty(self):
        return self.head is None
    def length(self):
        length = 0
        # If list is empty
        if self.head is None:
            return 0
        self.current = self.head.next
        while self.current != self.head:
            length += 1
            self.current = self.current.next
        return length
    # insert link at the first location
    def insert_first(self, key, data):
        # create a link
        new_node = Node(key, data)
        if self.is_empty():
            self.head = new_node
            self.head.next = self.head
        else:
            # point it to old first node
            new_node.next = self.head
            # point first to new first node
            self.head = new_node
    # delete first item
    def delete_first(self):
        # save reference to first link
        if self.head.next == self.head:
            temp_link = self.head
            self.head = None
            return temp_link
        # mark next to first link as first
        temp_link = self.head
        self.head = self.head.next
        # return the deleted link
        return temp_link
    # Diplay the list
    def print_list(self):
        ptr = self.head
        print("[", end=" ")
        # start from the beginning
        if self.head is not None:
            while ptr.next != ptr:
                print("({}, {})".format(ptr.key, ptr.data), end=" ")
                ptr = ptr.next
        print("]")
# Main function
if __name__ == '__main__':
    linked_list = LinkedList()
    linked_list.insert_first(1, 10)
    linked_list.insert_first(2, 20)
    linked_list.insert_first(3, 30)
    linked_list.insert_first(4, 1)
    linked_list.insert_first(5, 40)
    linked_list.insert_first(6, 56)
    print("Original List: ", end="")
    linked_list.print_list()
    while not linked_list.is_empty():
        temp = linked_list.delete_first()
        print("\nDeleted value: ({}, {})".format(temp.key, temp.data))
    # print list
    print("List after deleting all items: ", end="")
    linked_list.print_list()

Original List: [ (6, 56) (5, 40) (4, 1) (3, 30) (2, 20) ]

Deleted value: (6, 56)

Deleted value: (5, 40)

Deleted value: (4, 1)

Deleted value: (3, 30)

Deleted value: (2, 20)Deleted value: (1, 10)
List after deleting all items: [ ]