C++ Decimal to Octal

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

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

    i = 0;
    while(a != 0)
    {
        b[i] = a%8;
        a = a/8;
        i++;
    }

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

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

    return 0;
}