Single linklist program in c++(only inserting and display function)

#include<iostream>
using namespace std;

struct node{
int data;
node *next;
};

class sll{
private:
         node *head,*tail;

public:
        sll(){
        head=0;
        tail=0;
        }

        insert(int mydata){
        node *tmp=new node;
        tmp->data=mydata;
        tmp->next=0;
         int select;
        cout<<"Select one of this option\n";
        cout<<"1.insert element at first position\n";
        cout<<"2.insert element at Any position\n";
        cout<<"3.insert element at last position\n";
        cin>>select;
            switch(select){
        case 1:if(head==0){
                    head=tmp;
                    tail=tmp;
                }else{
                tmp->next=head;
                head=tmp;
                }
               break;

        case 2:int cont;
               if(head==0 && tail==0){
                 cout<<"please enter the no. at begin or end first\n";
                 }
                  else{
                        int inst;
                        tail=head;
                        cout<<"Enter the element after you want inserting\n";
                        cin>>inst;
                   while(tail->data!=inst)
                {
                    cont=1;
                    tail=tail->next;
                }
                tmp->next=tail->next;
                tail->next=tmp;
                }
                if(cont==0){
                 cout<<"plzzz enter correct element\n";
                }
               break;


        case 3:if(tail==0){
                 tail=tmp;
                 head=tmp;
                }else{
                while(tail->next!=0)
                {
                    tail=tail->next;
                }
                tail->next=tmp;
                tmp->next=0;
                }
                break;

     default : cout<<"wrong output\n";

            }
        }


      void display()
    {
        node *tmp=head;
        if(tmp == NULL)
        {
            cout << "NULL" << endl;
        }
        else
        {
            while(tmp!=0){
             cout << tmp->data << endl;
             tmp=tmp->next;
        }
        }
    }


};

int main()
{
   sll a;
   char again;
   int select,input;
   do{
   cout<<"Enter the given inputs \n";
   cout<<"1.Insert the input\n";
   cout<<"2.display element\n";
   cin>>select;
   switch(select){

   case 1:cout<<"Enter the element\n";
          cin>>input;
          a.insert(input);
          break;

  case 2:cout<<"Your outputs\n";
        a.display();
        break;

default:cout<<"your select wrong output\n";
  }
   cout<<"Do you want to run again yes press y or Y (or) no\n";
   cin>>again;

   }while(again=='y'||again=='Y');

   return 0;
}