C++ Decimal to Binary

#include<iostream>
using namespace std;
int main()
{
    int i, j, a, b[100] = {0};
    unsigned long long int s = 0;

    cout << "Enter Decimal Number : ";
    cin >> a;
    cout << endl;

    i = 0;
    while(a != 0)
    {
        b[i] = a%2;
        a = a/2;
        i++;
    }

    for(j=i-1;j>=0;j--)
    {
        s = s*10 + b[j];
    }

    cout << "Binary Number is :: " << s << endl;

    return 0;
}