Problem 48, the next challenge in Project Euler when they're sorted by ascending difficulty, seemed a bit familiar. It asks you to come up with a very large number and then report back the last few digits thereof. It reads:

The series, 11 + 22 + 33 + ... + 1010 = 10405071317.

Find the last ten digits of the series, 11 + 22 + 33 + ... + 10001000.

.NET makes this kind of problem extremely easy, as all one has to do is use BigIntegers to add all members of the series together. Taking that approach, I came up with the following:

static void Main(string[] args)
{
    BigInteger ridiculouslyHugeSum = new BigInteger(1);

    for (int i = 2; i <= 1000; i++)
        ridiculouslyHugeSum += BigIntPower(i, i);

    string sumString = ridiculouslyHugeSum.ToString();

    Console.WriteLine(sumString.Substring(sumString.Length - 10));
}

private static BigInteger BigIntPower(int exponentBase, int power)
{
    BigInteger exponentValue = new BigInteger(exponentBase);
    for (int j = 1; j < power; j++)
        exponentValue *= exponentBase;

    return exponentValue;
}

It worked fine; it just took a bit longer than I would have liked (read: a few seconds rather than a few milliseconds). Because the problem only asked for the ten least significant digits, I inserted two lines of code into the above that instructed the computer to lop off the most significant digit whenever it encountered an eleven digit number.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace Problem048
{
    class Program
    {
        static void Main(string[] args)
        {
            long runningTotal = 1;

            for (int i = 2; i <= 1000; i++)
            {
                runningTotal += lastTenOfPower(i, i);
                if (runningTotal > 10000000000)
                    runningTotal %= 10000000000;
            }

            Console.WriteLine(runningTotal);
        }

        private static long lastTenOfPower(int exponentBase, int power)
        {
            long exponentValue = exponentBase;
            for (int j = 1; j < power; j++)
            {
                exponentValue *= exponentBase;

                if (exponentValue > 10000000000)
                    exponentValue %= 10000000000;
            }

            return exponentValue;

        }
    }
}