Euler 44

Euler.cs

namespace Euler_solutions{
internal class Program{
private static void Main(string[] args)
{
	Stopwatch sw = new Stopwatch(); sw.Start();	// for timing
	long answer = 0;
	// Double Dictionary for checking pentagonals and the values
	Dictionary<long, long> pentagonals = new Dictionary<long, long>();
	Dictionary<long, long> pentagonalsVals = new Dictionary<long, long>();
	long i = 1;
	while (true)
	{	// determine current pentagonal number, and add these to both dictionaries
		long curPentagonal = i * (3 * i - 1) / 2;
		pentagonals.Add(i, curPentagonal);
		pentagonalsVals.Add(curPentagonal, i);
		for (long k = 1; k < i; k++)
		{	// Pj + Pk = Pi; Subtracting Pi with Pk gives Pj
			long PjVal = curPentagonal - pentagonals[k];
			if (pentagonalsVals.ContainsKey(PjVal) // SUM = OK
				&& pentagonalsVals.ContainsKey(pentagonals[k] - PjVal)) // DIFF = OK
			{	// PjVal is a pentagonal number; and the difference is also pentagonal
				answer = pentagonals[k] - PjVal;
				Console.WriteLine("Pj=" + pentagonalsVals[PjVal] + ", Pk=" + k + ", Psum=" + i + ", Pdiff=" + pentagonalsVals[pentagonals[k] - PjVal]);
				// ouput should be: Pj=1020, Pk=2167, Psum=2395, Pdiff=1912
				break;
			}
		}
		if (answer != 0) { break; }	// answer = found
		i++;
	}
	sw.Stop();
	Console.WriteLine("Answer: " + answer); // Answer Euler 44: 5482660 , runs in 126 ms
	Console.WriteLine("Runtime in miliseconds: " + sw.ElapsedMilliseconds);
	Console.ReadKey();
}
}
}