Project Euler Problem #45 Solution

euler-45.cs

public static void Main(string[] args)
{
    for (var n = 286L; ; n++)
    {
        var triangleNumber = TriangleNumber(n);
        if (IsPentagonal(triangleNumber) && IsHexagonal(triangleNumber))
        {
            Console.WriteLine(triangleNumber);
            break;
        }
    }
    Console.ReadKey();
}

public static bool IsPentagonal(long triangleNumber)
{
    var pentagonal = 0L;
    for (var n = 1L; pentagonal <= triangleNumber; n++)
    {
        pentagonal = n * (3 * n - 1) / 2;
        if (pentagonal == triangleNumber)
            return true;
    }
    return false;
}

public static bool IsHexagonal(long triangleNumber)
{
    var hexagonal = 0L;
    for (var n = 1L; hexagonal <= triangleNumber; n++)
    {
        hexagonal = n * (2 * n - 1);
        if (hexagonal == triangleNumber)
            return true;
    }
    return false;
}

private static long TriangleNumber(long n)
{
    return n * (n + 1) / 2;
}