#include <stack>
#include <cmath>
#include <iostream>

int main()
{
    unsigned long long int n;
    std::stack<unsigned long int> numbers;

    while (!std::cin.eof())
        numbers.push(n);
    while (!numbers.empty())
    {
        std::cout << sqrt( double( numbers.top() ) ) << " ";
        numbers.pop();
    }

    return 0;
}


Probably the stack this code builds is too large, but I can't see how to optimise it, given the sample numbers.

I don't think your first while loop ever terminates.