binary search in linked list

#include<bits/stdc++.h>
#define lli long long int
#define el  endl
#define oo  cout
#define ii  cin
#define FOR(i,a,b,c) for (int i = (a); i < (b); i=i+(c))


using namespace std;

class node{
    public:
lli   value;
node *next;

node(lli v){
value=v;
next=NULL;

}};

node *head=NULL,*tail=NULL,*tem;

void insert_at_back(lli v){

node *p=new node(v);

if(head==NULL){
    head=p;
    tail=p;
    return;
}

tem=head;
while(tem->next!=NULL){
    tem=tem->next;
}

tem->next=p;
tail=p;

}


void display(){

tem=head;
while(tem!=NULL){

    oo<<tem->value<<" ";
    tem=tem->next;
}
oo<<el;

}

node *middle(node *f,node *l){


node *mid=f;

while(f!=l){
    f=f->next;
    if(f!=l){
        f=f->next;
        mid=mid->next;
    }
}

return mid;

}


void Binary_Search(){
oo<<"Search Item:";
lli x;
ii>>x;

node *first=head;
node *last=tail;
node *mid=middle(first,last);

while(first!=last){

    if(mid->value == x){
        oo<<x<<" is Found !"<<el;
        return;
    }
    else if(mid->value > x){
        last=mid;
        mid=middle(first,last);
    }
    else if(mid->value < x){
        first=mid->next;
        mid=middle(first,last);
    }

}

oo<<x<<" Not Found!"<<el;

}

int main()
{
lli n,v;
oo<<"Number: ";
ii>>n;

oo<<"Numbers: ";
FOR(i,0,n,1){
ii>>v;
insert_at_back(v);
}
display();

Binary_Search();

return 0;
}