C语言实现数据结构 - 双向链表

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概览

双向链表是链表的一种变体，与单向链表相比，它可以方便地向前和向后导航。以下是一些理解双向链表概念的重要术语。

  • 节点 − 链表的每个节点可以存储一个称为元素的数据。

  • Next − 链表的每个节点都包含一个指向下一个节点的链接，称为 Next。

  • Prev − 链表的每个节点都包含一个指向前一个节点的链接，称为 Prev。

  • 链表 − 链表包含指向第一个节点（称为 First）和最后一个节点（称为 Last）的连接链接。

双向链表表示

Doubly Linked List

根据上面所示的插图，以下是要考虑的重要事项。

  • 双向链表包含一个称为 first 和 last 的节点元素。

  • 每个节点都包含一个或多个数据字段和一个称为 next 的链接字段。

  • 每个节点都使用其 next 链接与其下一个节点链接。

  • 每个节点都使用其 prev 链接与其前一个节点链接。

  • 最后一个节点包含一个空链接，以标记列表的结尾。

基本操作

以下是列表支持的基本操作。

  • 插入− 在列表开头添加一个元素。

  • 删除 − 删除列表开头的元素。

  • 插入到末尾− 在列表末尾添加一个元素。

  • 删除末尾元素 − 删除列表末尾的元素。

  • 插入到指定元素之后 − 在列表中的某个元素之后添加一个元素。

  • 删除 − 使用键从列表中删除一个元素。

  • 正向显示 − 以正向方式显示整个列表。

  • 反向显示 − 以反向方式显示整个列表。

插入操作

以下代码演示了在双向链表开头进行插入操作。

//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()){
      //make it the last link
      last = link;
   } else {
      //update first prev link
      head->prev = link;
   }
   //point it to old first link
   link->next = head;
   
   //point first to new first link
   head = link;
}

删除操作

以下代码演示了在双向链表开头进行删除操作。

//delete first item
struct node* deleteFirst(){
   //save reference to first link
   struct node *tempLink = head;
   
   //if only one link
   if(head->next == NULL){
      last = NULL;
   } else {
      head->next->prev = NULL;
   }
   head = head->next;
   
   //return the deleted link
   return tempLink;
}

在末尾插入操作

以下代码演示了在双向链表末尾进行插入操作。

//insert link at the last location
void insertLast(int key, int data){
   //create a link
   struct node *link = (struct node*) malloc(sizeof(struct node));
   link->key = key;
   link->data = data;
   if(isEmpty()){
      //make it the last link
      last = link;
   } else {
      //make link a new last link
      last->next = link;     
      
      //mark old last node as prev of new link
      link->prev = last;
   }
   //point last to new last node
   last = link;
}

示例

DoublyLinkedListDemo.c

在线演示 
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <stdbool.h>

struct node {
   int data;
   int key;
   struct node *next;
   struct node *prev;
};
//this link always point to first Link 
struct node *head = NULL;

//this link always point to last Link 
struct node *last = NULL;
struct node *current = NULL;

//is list empty
bool isEmpty(){
   return head == NULL;
}
int length(){
   int length = 0;
   struct node *current;
   for(current = head; current!=NULL;
      current = current->next){
      length++;
   }
   return length;
}
//display the list in from first to last
void displayForward(){
   //start from the beginning
   struct node *ptr = head;
   
   //navigate till the end of the list
   printf("\n[ ");
   while(ptr != NULL){        
      printf("(%d,%d) ",ptr->key,ptr->data);
      ptr = ptr->next;
   }
   printf(" ]");
}
//display the list from last to first
void displayBackward(){
   //start from the last
   struct node *ptr = last;
   
   //navigate till the start of the list
   printf("\n[ ");
   while(ptr != NULL){    
      //print data
      printf("(%d,%d) ",ptr->key,ptr->data);
      
      //move to next item
      ptr = ptr ->prev;
      printf(" ");
   }
   printf(" ]");
}
//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()){
      //make it the last link
      last = link;
   } else {
      //update first prev link
      head->prev = link;
   }
   //point it to old first link
   link->next = head;
   
   //point first to new first link
   head = link;
}
//insert link at the last location
void insertLast(int key, int data){
   //create a link
   struct node *link = (struct node*) malloc(sizeof(struct node));
   link->key = key;
   link->data = data;
   if(isEmpty()){
      //make it the last link
      last = link;
   } else {
      //make link a new last link
      last->next = link;     
      
      //mark old last node as prev of new link
      link->prev = last;
   }
   //point last to new last node
   last = link;
}
//delete first item
struct node* deleteFirst(){
   //save reference to first link
   struct node *tempLink = head;
   
   //if only one link
   if(head->next == NULL){
      last = NULL;
   } else {
      head->next->prev = NULL;
   }
   head = head->next;
   //return the deleted link
   return tempLink;
}
//delete link at the last location
struct node* deleteLast(){
   //save reference to last link
   struct node *tempLink = last;
   
   //if only one link
   if(head->next == NULL){
      head = NULL;
   } else {
      last->prev->next = NULL;
   }
   last = last->prev;
   //return the deleted link
   return tempLink;
}
//delete a link with given key
struct node* delete(int key){
   //start from the first link
   struct node* current = head;
   struct node* previous = NULL;
   
   //if list is empty
   if(head == NULL){
      return NULL;
   }
   //navigate through list
   while(current->key != key){
      //if it is last node
      if(current->next == NULL){
         return NULL;
      } else {
         //store reference to current link
         previous = current;
         
         //move to next link
         current = current->next;             
      }
   }
   //found a match, update the link
   if(current == head) {
      //change first to point to next link
      head = head->next;
   } else {
      //bypass the current link
      current->prev->next = current->next;
   }    
   if(current == last){
      //change last to point to prev link
      last = current->prev;
   } else {
      current->next->prev = current->prev;
   }
   return current;
}
bool insertAfter(int key, int newKey, int data){
   //start from the first link
   struct node *current = head;      
   
   //if list is empty
   if(head == NULL){
      return false;
   }
   //navigate through list
   while(current->key != key){
      //if it is last node
      if(current->next == NULL){
         return false;
      } else {           
         //move to next link
         current = current->next;             
      }
   }
   //create a link
   struct node *newLink = (struct node*) malloc(sizeof(struct node));
   newLink->key = key;
   newLink->data = data;

   if(current==last) {
      newLink->next = NULL; 
      last = newLink; 
   } else {
      newLink->next = current->next;         
      current->next->prev = newLink;
   }
   newLink->prev = current; 
   current->next = newLink; 
   return true; 
}
main() {
   insertFirst(1,10);
   insertFirst(2,20);
   insertFirst(3,30);
   insertFirst(4,1);
   insertFirst(5,40);
   insertFirst(6,56); 

   printf("\nList (First to Last): ");  
   displayForward();
   printf("\n");
   printf("\nList (Last to first): "); 
   displayBackward();

   printf("\nList , after deleting first record: ");
   deleteFirst();        
   displayForward();

   printf("\nList , after deleting last record: ");  
   deleteLast();
   displayForward();

   printf("\nList , insert after key(4) : ");  
   insertAfter(4,7, 13);
   displayForward();

   printf("\nList  , after delete key(4) : ");  
   delete(4);
   displayForward();
}

输出

如果我们编译并运行以上程序，它将产生以下输出：

List (First to Last): 
[ (6,56) (5,40) (4,1) (3,30) (2,20) (1,10) ]

List (Last to first): 
[ (1,10) (2,20) (3,30) (4,1) (5,40) (6,56) ]
List , after deleting first record: 
[ (5,40) (4,1) (3,30) (2,20) (1,10) ]
List , after deleting last record: 
[ (5,40) (4,1) (3,30) (2,20) ]
List , insert after key(4) : 
[ (5,40) (4,1) (4,13) (3,30) (2,20) ]
List , after delete key(4) : 
[ (5,40) (4,13) (3,30) (2,20) ]
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