class Node{
public:
  int data;
  Node *next;
  Node(int da=0,Node *p=NULL){
    this->data=da;
    this->next=p;
  }
};

class List{
private:
  Node *head,*tail;
  int position;
public:
  List(){head=tail=NULL;};
  ~List(){delete head;delete tail;};
  void print();
  void Insert(int da=0);
  void Delete(int da=0);
  void Search(int da=0);
};

#include<iostream>
using namespace std;

class Node{
public:
  int data;
  Node *next;
  Node(int da=0,Node *p=NULL){
    this->data=da;
    this->next=p;
  }
};

class List{
private:
  Node *head,*tail;
  int position;
public:
  List(){head=tail=NULL;};
  ~List(){delete head;delete tail;};
  void print();
  void Insert(int da=0);
  void Delete(int da=0);
  void Search(int da=0);
  int getValueAt(int position);
  void setValueAt(int position,int da);
};

int List::getValueAt(int position){
  Node *p=head;
  if(p==NULL){
    cout<<"The List is Empty!"<<endl;
  }else{
    int posi=0;
    while(p!=NULL&&posi!=position){
      posi++;
      p=p->next;
    }
    if(p==NULL){
      cout<<"There is no value of this position in this List!"<<endl;
    }else{
      cout<<"In this Position,the value is"<<p->data<<endl;
    }
  }
  return p->data;
}

void List::setValueAt(int position,int da){
  Node *p=head;
  if(p==NULL){
    cout<<"The List is Empty!"<<endl;
  }else{
    int posi=0;
    while(p!=NULL&&posi!=position){
      posi++;
      p=p->next;
    }
    if(p==NULL){
      cout<<"There is No Position in this List!"<<endl;
    }else{
      p->data=da;
      cout<<"The Value in this position has been Updated!"<<endl;
    }
  }
}

void List::Search(int da){

Node *p=head;
  if(p==NULL){
    cout<<"Sorry, The List is Empty!"<<endl;
    return;
  }
  int count=0;
  while(p!=NULL&&p->data!=da){
    p=p->next;
    count++;
  }
  cout<<"the value you want to search is at position %d"<<count<<endl;
}

void List::Delete(int da){
  Node *p=head,*q=head;
  if(p==NULL){
    cout<<"Sorry, The List is Empty!"<<endl;
    return;
  }
  while(p!=NULL&&p->data!=da){
    q=p;
    p=p->next;
  }
  q->next=p->next;
  cout<<"The Deletion Operation had been finished!"<<endl;
}

void List::Insert(int da){
  if(head==NULL){
    head=tail=new Node(da);
    head->next=NULL;
    tail->next=NULL;
  }else{
    Node *p=new Node(da);
    tail->next=p;
    tail=p;
    tail->next=NULL;
  }

}

void List::print(){
  Node *p=head;
  while(p!=NULL){
    cout<<p->data<<" \a";
    p=p->next;
  }
  cout<<endl;
}

int main(){
  cout<<"Hello World!"<<endl;
  List l1;
  l1.Insert(1);
  l1.Insert(2);
  l1.Insert(3);
  l1.Insert(4);
  l1.Insert(5);
  l1.Insert(6);
  l1.Insert(7);
  l1.print();
  l1.Search(4);
  l1.Delete(6);
  l1.print();
  l1.getValueAt(3);
  l1.setValueAt(3,9);
  l1.print();
  cout<<"The End!"<<endl;
  return 0;
}

//在此我想解释的是，之所以数字4在链表中的位置为3，是因为其是从零开始计数的