stack sort without loop

#include <iostream>
using namespace std;

int index =-1;
int number[100];
int size=100;
void push(int x){

    if(index+1==size){
        cout<<"overFlow\n";
        }
    else{
    index++;
    number[index] = x;
}
    }

int top(){


return number[index];
    }

void pop(){

    if(index<0)
        cout<<"underflow";
    else
    index-=1;

    }



void display(){

    for(int i=0;i<=index;i++)
        cout<<number[i]<<" ";
        cout<<endl;

    }
bool empty(){

    if (index ==-1)
        return true;

    return false;

    }

void sortInsert(int x){

    if(empty() || x>top()){

        push(x);
        return;

        }
    int p = top();
    pop();
    sortInsert(x);
    push(p);

    }

void sortStack(){
    if( !empty()){

        int x = top();
        pop();
        sortStack();
        //cout<<x<<" ";
        sortInsert(x);
        //cout<<x<<" ";
        }
    }

int main(int argc, char **argv)
{

    push(100);
    push(20);
    push(-10);
    push(1);
    push(2);

    sortStack();
    display();



    return 0;
}