# Encapsulates a double linked list

In this blog, I will try to encapsulate a double linked list.

# Definitions

In computer science, a doubly linked list is a linked data structure that consists of a set of sequentially linked records called nodes.

# Function

This double linked list contains these functions:

  1. Getting the size of the list.
  2. Checking the list whether empty or not.
  3. Inserting element in anywhere in the list.
  4. Deleting element in anywhere in the list.
  5. Getting element in anywhere in the list.
  6. Showing the whole list.
  7. Clearing the whole list.
  8. Using the iterator to traverse the whole list.

# Method

I define a class named DoubleLink. In the class, it has a private class about node , properties, interface, and iterator.

# node class

The object of this class is the element in DoubleLink. The node class has three properties. one property which stores data is generic paradigm. The other two properties are pointers which point to the next or previous element.

let us look the code:

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private:	
class node{//the class of node
public:
T data;
node *prev;
node *next;
};

# Iterator

I define a Iterator class to traverse the whole elements of the list.
By overloading the operator, I can traverse the list by ++ instead of by ->next;

let us look the code:

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public:
class Iterator {
private:
node *prv;

public:
Iterator(node* p = nullptr) {
prv = p;
}

~Iterator(){
if(prv!=NULL)
delete prv;
}

Iterator& operator ++ () { //Overload the autoincrement symbol
prv = prv->next;
return *this;
}

Iterator& operator = (Iterator x){
prv=x.prv;
return *this;
}

Iterator& operator -- () {
prv = prv->prev;
return *this;
}

T operator *() const{
return prv->data;
}

bool operator != (const Iterator& rhs) const{
return (rhs.prv != this->prv);
}
bool operator == (const Iterator& rhs) const{
return (rhs.prv == this-> prv);
}
};

# Property

Because the List need some variables to express the properties, I define some variable.

  1. *head :a pointer point the head of list
  2. *tail :a pointer point the tail of list
  3. len :the size of the list
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    private:
    node *head;//the pointer point to the head;
    node *tail;//the pointer point to the tail;
    int len;//the length of link

# Interface

I define these interfaces to realize the functions of the List;

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public:
DoubleLink();
DoubleLink(const DoubleLink &l);
~DoubleLink();

Iterator begin()
{
return Iterator(head);
}
Iterator end()
{
return Iterator(tail->next);
}

int size();//get the size of link

bool IsEmpty();//judge whether the link is empty

void insert(int index,T data);
void pushBack(T data);
void pushHead(T data);

T get(int index);//find a node

void popBack();//delete the end of link
void deletenode(int index);//delete a node
void popHead();//delete the head of link

void show();//print all nodes
void clear();//delete all of the Link

# Constructor and Copy Constructor

Through the constructor, the properties of list has a initial value.

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template<class T>
DoubleLink<T>::DoubleLink()
{
head = NULL;
tail =NULL;
len =0;
}
```
Through the copy constructor, we can assign a DoubleList object to the others
```C++
template<class T>
DoubleLink<T>::DoubleLink(const DoubleLink &l)
{
if(l.head!=NULL)
{
node *tmp=l.head;
head = new node();
head ->data=tmp->data;
len++;
tmp=tmp->next;
node *temp = head;
while(tmp!=l.tail)
{
temp->next = new node();
len++;
temp->data = tmp->data;
tmp = tmp->next;
}
}
else
{
head=tail=NULL;
len = 0;
}
}

# Destructor

Through the destructor ,we delete all nodes in the list to avoid memory leak.

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template<class T>
DoubleLink<T>::~DoubleLink()
{
node *tmp=head;
while(head!=tail)
{
head=head->next;
delete tmp;
tmp = head;
}
len = 0;
}

# getsize()

We can get the number of nodes in the list

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template<class T>
int DoubleLink<T>::size()
{
return len;
}

# IsEmpty()

We can check the list whether empty or not

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template<class T>
bool DoubleLink<T>::IsEmpty()
{
return len==0;
}

# insert() , pushBack() , pushHead()

We use these function to insert the element to the list.

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template<class T>
void DoubleLink<T>::insert(int index,T data)
{
node *p,*q;
p= head;
q=head->next;
if(index>len||index<0)
cout<<"you can't add the node,because the index you entered is wrong"<<endl;
else
{
for(int i=2;i<index;++i)
{
p=p->next;
q=q->next;
}
p->next = new node();
p->next->data= data;
p->next->prev=p;

q->prev=p->next;
q->prev->next=q;
}

}

template<class T>
void DoubleLink<T>::pushBack(T data)
{
if(head==NULL)
{
head = new node();
head->data = data;
len++;
tail=head;
}
else
{
tail->next=new node();
tail->next->data= data;
len++;
tail->next->prev=tail;
tail=tail->next;
tail->next=NULL;
}
}

template<class T>
void DoubleLink<T>::pushHead(T data)
{
if(head==NULL)
{
head = new node();
head->data = data;
len++;
tail=head;
}
else
{
head->prev=new node();
head->prev->data= data;
len++;
head->prev->next=head;
head=head->prev;
}
}

# get()

This function can return data of node.

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template<class T>
T DoubleLink<T>::get(int index)
{
node *p;
p=head;
if(index<0||index>len)
{
cout<<"you can't get it.because you enter a wrong index"<<endl;
return NULL;
}
else
{
for(int i=1;i<index;++i)
{
p=p->next;
}
return p->data;
}
}

# deletenode() ,popBack(), popHead()

These functions can delete elements in lists.

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template<class T>
void DoubleLink<T>::deletenode(int index)
{
node *p,*q;
p=head;
q=head->next;
if(index==1)
{
head=head->next;
delete head->prev;
return;
}
if(index==len)
{
tail=tail->prev;
delete tail->next;
return;
}
if(index>len||index<0)
cout<<"you can't delete the node,because the index you entered is wrong"<<endl;
else
{
for(int i=2;i<index;++i)
{
p=p->next;
q=q->next;
}
p->next= q->next;
q->next->prev=p;
delete q;
}
}

template<class T>
void DoubleLink<T>::popBack()
{
if(len==0)
cout<<"the link is empty"<<endl;
else
{
tail=tail->prev;
delete tail->next;
tail->next=NULL;
}
}

template<class T>
void DoubleLink<T>::popHead()
{
if(len==0)
cout<<"the link is empty"<<endl;
else
{
head=head->next;
delete head->prev;
}

}

# show()

Showing the whole list.

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template<class T>
void DoubleLink<T>::show()
{
node *p;
p= head;
if(p==NULL)
{
cout<<"Link is empty"<<endl;
}
else
{
while(p!=tail->next)
{
cout<<p->data<<" ";
p=p->next;
}
cout<<endl;
}
}
```

###### clear()
Remove the list.
```C++
template<class T>
void DoubleLink<T>::clear()
{
node *p;
p=head->next;
while(p!=tail)
{
p=p->next;
delete p->prev;
}
delete head;
delete tail;
tail=NULL;
head=NULL;
len=0;
}

# The Whole Code

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/*
功能:
1.大小
2.判断空链表
3.首位置,末位置,任何位置添加
4.首位置,末位置,任何位置删除
5.查询
6.展示所有元素
7.清空链表
8.迭代器遍历链表

*/

#include<iostream>
#include<cstdio>
using namespace std;
template<class T>

class DoubleLink//the class of Double link
{


private:
class node{//the class of node
public:
T data;
node *prev;
node *next;
};
public:
class Iterator { //简单版迭代器
private:
node *prv;

public:
Iterator(node* p = nullptr) {
prv = p;
}

~Iterator(){
if(prv!=NULL)
delete prv;
}

Iterator& operator ++ () { //重载前置自增符号
prv = prv->next;
return *this;
}

Iterator& operator = (Iterator x){
prv=x.prv;
return *this;
}

Iterator& operator -- () { //重载前置自增符号
prv = prv->prev;
return *this;
}

T operator *() const{
return prv->data;
}

bool operator != (const Iterator& rhs) const{
return (rhs.prv != this->prv);
}
bool operator == (const Iterator& rhs) const{
return (rhs.prv == this-> prv);
}
};



private:
node *head;//the pointer point to the head;
node *tail;//the pointer point to the tail;
int len;//the length of link

public:
DoubleLink();
DoubleLink(const DoubleLink &l);
~DoubleLink();

Iterator begin()
{
return Iterator(head);
}
Iterator end()
{
return Iterator(tail->next);
}

int size();//get the size of link

bool IsEmpty();//judge whether the link is empty

void insert(int index,T data);
void pushBack(T data);
void pushHead(T data);

T get(int index);//find a node

void popBack();//delete the end of link
void deletenode(int index);//delete a node
void popHead();//delete the head of link

void show();//print all nodes

void clear();//delete all of the Link

};


template<class T>
DoubleLink<T>::DoubleLink()
{
head = NULL;
tail =NULL;
len =0;
}

template<class T>
DoubleLink<T>::DoubleLink(const DoubleLink &l)
{
if(l.head!=NULL)
{
node *tmp=l.head;
head = new node();
head ->data=tmp->data;
len++;
tmp=tmp->next;
node *temp = head;
while(tmp!=l.tail)
{
temp->next = new node();
len++;
temp->data = tmp->data;
tmp = tmp->next;
}
}
else
{
head=tail=NULL;
len = 0;
}
}

template<class T>
DoubleLink<T>::~DoubleLink()
{
node *tmp=head;
while(head!=tail)
{
head=head->next;
delete tmp;
tmp = head;
}
len = 0;
}

template<class T>
int DoubleLink<T>::size()
{
return len;
}

template<class T>
bool DoubleLink<T>::IsEmpty()
{
return len==0;
}

template<class T>
void DoubleLink<T>::insert(int index,T data)
{
node *p,*q;
p= head;
q=head->next;
if(index>len||index<0)
cout<<"you can't add the node,because the index you entered is wrong"<<endl;
else
{
for(int i=2;i<index;++i)
{
p=p->next;
q=q->next;
}
p->next = new node();
p->next->data= data;
p->next->prev=p;

q->prev=p->next;
q->prev->next=q;
}

}

template<class T>
void DoubleLink<T>::pushBack(T data)
{
if(head==NULL)
{
head = new node();
head->data = data;
len++;
tail=head;
}
else
{
tail->next=new node();
tail->next->data= data;
len++;
tail->next->prev=tail;
tail=tail->next;
tail->next=NULL;
}
}

template<class T>
void DoubleLink<T>::pushHead(T data)
{
if(head==NULL)
{
head = new node();
head->data = data;
len++;
tail=head;
}
else
{
head->prev=new node();
head->prev->data= data;
len++;
head->prev->next=head;
head=head->prev;
}
}

template<class T>
T DoubleLink<T>::get(int index)
{
node *p;
p=head;
if(index<0||index>len)
{
cout<<"you can't get it.because you enter a wrong index"<<endl;
return NULL;
}
else
{
for(int i=1;i<index;++i)
{
p=p->next;
}
return p->data;
}
}

template<class T>
void DoubleLink<T>::deletenode(int index)
{
node *p,*q;
p=head;
q=head->next;
if(index==1)
{
head=head->next;
delete head->prev;
return;
}
if(index==len)
{
tail=tail->prev;
delete tail->next;
return;
}
if(index>len||index<0)
cout<<"you can't delete the node,because the index you entered is wrong"<<endl;
else
{
for(int i=2;i<index;++i)
{
p=p->next;
q=q->next;
}
p->next= q->next;
q->next->prev=p;
delete q;
}
}

template<class T>
void DoubleLink<T>::popBack()
{
if(len==0)
cout<<"the link is empty"<<endl;
else
{
tail=tail->prev;
delete tail->next;
tail->next=NULL;
}
}

template<class T>
void DoubleLink<T>::popHead()
{
if(len==0)
cout<<"the link is empty"<<endl;
else
{
head=head->next;
delete head->prev;
}

}


template<class T>
void DoubleLink<T>::show()
{
node *p;
p= head;
if(p==NULL)
{
cout<<"Link is empty"<<endl;
}
else
{
while(p!=tail->next)
{
cout<<p->data<<" ";
p=p->next;
}
cout<<endl;
}
}

template<class T>
void DoubleLink<T>::clear()
{
node *p;
p=head->next;
while(p!=tail)
{
p=p->next;
delete p->prev;
}
delete head;
delete tail;
tail=NULL;
head=NULL;
len=0;
}

int main()
{
int n,m;
DoubleLink<int>* lin=new DoubleLink<int>();

if(lin->IsEmpty())
{
cout<<"空链表"<<endl;
}
else
{
cout<<"链表非空"<<endl;
}
cout<<"插入节点:"<<endl;
lin->pushBack(5);
lin->pushBack(8);
lin->pushBack(7);
lin->pushBack(4);
lin->pushBack(3);
lin->pushBack(4);
lin->pushBack(1);
lin->pushBack(0);

cout<<"链表大小"<<endl;
cout<<lin->size()<<endl;

cout<<"输出所有节点:"<<endl;
lin->show();

if(lin->IsEmpty())
{
cout<<"空链表"<<endl;
}
else
{
cout<<"链表非空"<<endl;
}


cout<<"输入要查找第几个节点:"<<endl;
cin>>n;
cout<<lin->get(n)<<endl;

cout<<"输入要删除第几个节点:"<<endl;
cin>>n;
lin->deletenode(n);
lin->show();

cout<<"头部添加节点:"<<endl;
cin>>n;
lin->pushHead(n);
lin->show();

cout<<"在任意位置添加结点:"<<endl;
cout<<"位置:"<<endl;
cin>>m;
cout<<"节点:"<<endl;
cin>>n;
lin->insert(m,n);
lin->show();

cout<<"头部删除节点:"<<endl;
lin->popHead();
lin->show();

cout<<"尾部删除节点:"<<endl;
lin->popBack();
lin->show();
cout<<"输出迭代器第一个"<<endl;
DoubleLink<int>::Iterator it=lin->begin();

cout<<"迭代器遍历"<<endl;
for(DoubleLink<int>::Iterator it=lin->begin();it!=lin->end();++it)
{
cout<<*it<<" ";
}
cout<<endl;
cout<<"清空节点:"<<endl;
lin->clear();
lin->show();

if(lin->IsEmpty())
{
cout<<"空链表"<<endl;
}
else
{
cout<<"链表非空"<<endl;
}

return 0;
}