preface

I wrote a blog before, which is a one-way linked list without taking the lead
Now I want to introduce the two-way leading circular linked list , Before the introduction, we know something

1. Headless unidirectional acyclic list ： Simple structure , Generally, it is not used to store data alone . In fact, it is more as a substructure of other data structures , Such as hash bucket 、 Adjacency table of graph and so on . In addition, this kind of structure appears a lot in the written interview .
2. Take the lead in two-way circular list ： The most complex structure , It's usually used to store data separately . Linked list data structure used in practice , Are the leading two-way circular list . In addition, although the structure is complex , But after using the code implementation, we will find that the structure will bring many advantages , Instead of
   Simple.
   
   It looks something like this , Although the structure is complex , But it's really simple to add, delete, check and change ！

Same old thing
This is a List.h Code for Declaration of functions

 #define _CRT_SECURE_NO_WARNINGS 1
#pragma once
#include<stdio.h>
#include<stdlib.h>
#include<assert.h>
typedef int LTDataType;
typedef struct ListNode
{
    
	LTDataType data;
	struct ListNode* prev;
	struct ListNode* next;
}LTN;
LTN* ListInit();// Initialization of double linked list 
LTN* ListBuyNode(LTDataType x);// Create a node 
void Listprint(LTN* phead);// Printing of double linked list 
void PushBack(LTN* phead, LTDataType x);// Tail insertion 
void PuchFront(LTN* phead, LTDataType x);// Head insertion 
void ListInsert(LTN* pos, LTDataType x);// stay pos Insert before 
void ListErase(LTN* pos);// Delete pos The node of position 
void PopFront(LTN* phead);// Head deletion 
void PopBack(LTN* phead);// Deletion at the end 
LTN* ListFind(LTN* phead, LTDataType x);// lookup x The location of 

The first is to write a function that creates a node
This will facilitate the next operation

LTN* ListBuyNode(LTDataType x)// Create a node 

{
    

	LTN* newnode = (LTN*)malloc(sizeof(LTN));
	if (newnode == NULL)
	{
    
		printf("malloc fail\n");
		exit(-1);
	}
	newnode->data = x;
	newnode->next = NULL;
	newnode->prev = NULL;
	return newnode;
}

Then is the initialization of the linked list

LTN* ListInit()// Initialization of double linked list 
{
    
	LTN* ret = ListBuyNode(0);
	ret->next = ret;
	ret->prev = ret;
	return ret;

}

Printing of linked list

void Listprint(LTN* phead)// Printing of double linked list 

{
    
	assert(phead);
	LTN* cur = phead->next;
	while (cur != phead)
	{
    
		printf("%d ", cur->data);
		cur = cur->next;
	}
	printf("\n\n");

}

Because there is no difficulty , The things to note have been mentioned in the previous sequence table and single chain table
So it's all directly on the code

increase

Head insertion , Tail insertion , stay pos Insert before
We just need to master the last function —— That is to say pos Just insert this function before , The other two functions call the third function directly

void ListInsert(LTN* pos, LTDataType x)// stay pos Insert before 
{
    
	assert(pos);
	LTN* ret = ListBuyNode(x);
	ret->next = pos;
	ret->prev = pos->prev;
	pos->prev->next = ret;
	pos->prev = ret;
}

void PushBack(LTN* phead, LTDataType x)// Tail insertion 
{
    
	assert(phead);
	ListInsert(phead, x);
}


void PuchFront(LTN* phead, LTDataType x)// Head insertion 

{
    
	assert(phead);
	ListInsert(phead->next, x);
}

Delete

Head deletion Deletion at the end Delete pos Location
Empathy Mainly master the third function

void ListErase(LTN* pos)// Delete pos The node of position 

{
    
	assert(pos);
	LTN* prev = pos->prev;
	LTN* next = pos->next;
	prev->next = next;
	next->prev = prev;
	/*pos->next->prev = pos->prev; pos->prev->next = pos->next;*/// Both are OK 
	free(pos);
	pos = NULL;

}

void PopFront(LTN* phead)// Head deletion 

{
    
	assert(phead);
	ListErase(phead->next);
}
void PopBack(LTN* phead)// Deletion at the end 
{
    
	assert(phead);
	ListErase(phead->prev);
}

check

The search here is only to search according to the numerical value , It's the same if you look it up by address

LTN* ListFind(LTN* phead, LTDataType x)// lookup x The location of 

{
    
	assert(phead);
	LTN* cur = phead->next;
	while (cur != phead)
	{
    
		if (cur->data == x)
			return cur;
		else
			cur = cur->next;
	}

	return NULL;
}

Comrades , The two-way linked list is here