双向带头循环链表：结构复杂，操作简单

0.结构体定义

这里方便浏览，特地没有将int类型重命名为TLDateType

#define _CRT_SECURE_NO_WARNINGS 1
#pragma once
#include<stdio.h>
#include<stdlib.h>
#include<assert.h>
#include<stdbool.h>
struct ListNode
{
  int val;
  struct ListNode* prev;
  struct ListNode* next;
};

1.初始化

1. 带头需要初始化一个哨兵位头结点
2. 传二级指针
3. 初始化自己的prev和next都指向自己

void ListInit(struct ListNode** pphead)
{
  struct ListNode* Guard = (struct ListNode*)malloc(sizeof(struct ListNode));
  if (Guard == NULL)
  {
    perror("ListInit");
    return;
  }
  *pphead = Guard;
  Guard->next = Guard;
  Guard->prev = Guard;
}

2.尾插

1. 一级指针
2. 直接prev找尾
3. 即是无首元结点，也可（头结点自环）

struct ListNode* BuyListNode(int x)
{
  struct ListNode* newnode = (struct ListNode*)malloc(sizeof(struct ListNode));
  if (newnode == NULL)
  {
    perror("BuySListNode");
    return NULL;
  }
  newnode->val = x;
  newnode->prev = NULL;
  newnode->next = NULL;
  return newnode;
}
void ListPushBack(struct ListNode* phead,int x)
{
  assert(phead);
  struct ListNode* newnode = BuyListNode(x);
  struct ListNode* tail = phead->prev;
  newnode->next = phead;
  phead->prev = newnode;
  tail->next = newnode;
  newnode->prev = tail;
}

3.打印

1. 可assert断言（建议单链表不用断言）---头结点
2. 起始条件cur=phead->next;
3. 循环终止条件cur==phead

void ListPrint(struct ListNode* phead)
{
  assert(phead);
  struct ListNode* cur = phead->next;
  while (cur != phead)
  {
    printf("%d->", cur->val);
    cur = cur->next;
  }
  printf("NULL\n");
}

4.头插

1. 一级指针
2. 即是无首元结点，也可（头结点自环）

void ListPushFront(struct ListNode* phead,int x)
{
  assert(phead);
  struct ListNode* newnode = BuyListNode(x);
  struct ListNode* next = phead->next;
  newnode->next = next;
  next->prev = newnode;
  phead->next = newnode;
  newnode->prev = phead;
}

5.任意位置插入前面位置

1. 因为有prev，前插不同phead
2. 尾插,头插就用ListInsert传不同pos参数
3. 如果pos传phead,就相当于于是尾插
4. 如果pos传phead->next，就相当于于是头插

void ListInsert(struct ListNode* pos, int x)
{
  assert(pos);
  struct ListNode* newnode = BuyListNode(x);
  struct ListNode* prev = pos->prev;
  prev->next = newnode;
  newnode->prev = prev;
  newnode->next = pos;
  pos->prev = newnode;
}
头插：ListNode(phead->next,x);
尾插:ListNode(phead,x);

6.尾删

1. 不为空，!ListEmpty(plhead)
2. prev,tail,phead

bool ListEmpty(struct ListNode* phead)
{
  return phead->next == phead;
}
void ListPopBack(struct ListNode* phead)
{
  assert(phead);
  assert(!ListEmpty(phead));
  struct ListNode* tail = phead->prev;
  struct ListNode* prev = tail->prev;
  prev->next = phead;
  phead->prev = prev;
  free(tail);
  tail = NULL;
}

7.头删

1. 不为空，!ListEmpty(plhead)
2. phead,first,second

void ListPopFront(struct ListNode* phead)
{
  assert(phead);
  assert(!ListEmpty(phead));
  struct ListNode* first = phead->next;
  struct ListNode* second = first->next;
  phead->next = second;
  second->prev = phead;
  free(first);
  first = NULL;
}

8.链表长度

1. 起始条件:cur=phead->next
2. 终止条件:cur!=phead;

size_t ListSize(struct ListNode* phead)
{
  size_t size = 0;
  struct ListNode* cur = phead->next;
  while (cur != phead)
  {
    ++size;
    cur = cur->next;
  }
  return size;
}

9.任意位置删除当前位置

1. 不为空，!ListEmpty(plhead)
2. 尾删，头删就用ListErase传不同pos参数
3. 如果pos传phead->prev，就是尾删
4. 如果pos传phead->next,就是头删

void ListErase(struct ListNode* pos)
{
  assert(pos);
  struct ListNode* prev = pos->prev;
  struct ListNode* next = pos->next;
  prev->next = next;
  next->prev = prev;
  free(pos);
  pos = NULL;
}

10. 销毁

1. 看是否保留哨兵头，来传一级或二级指针
2. 先保留哨兵头作为判断循环条件，最后决定是否释放哨兵头

void ListDestory(struct ListNode** pphead)
{
  assert(pphead);
  struct ListNode* cur = (*pphead)->next;
  while (cur != *pphead)
  {
    struct ListNode* next = cur->next;
    free(cur);
    cur = next;
  }
  free(*pphead);
  *pphead = NULL;
}