使用 C 的 DSA - 循环链表

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概述

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

循环双向链表

根据上图所示，以下是需要考虑的要点。

• 在单链表和双链表两种情况下，Last Link'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 prev 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。

• 删除- 从列表的开头插入一个元素。

• 显示- 显示列表。

长度操作

以下代码演示了基于单链表的循环链表中的插入操作。

//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;
   }
}

删除操作

下面的代码演示了基于单链表的循环链表中的删除操作。

//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;
}

显示列表操作

以下代码演示了循环链表中的显示列表操作。

//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(" ]");
}

例子

双链表Demo.c

#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(" ]");
}
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();   
}

输出

如果我们编译并运行上面的程序，那么它将产生以下输出 -

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: 
[ ]