1. 設計一程式能找出四位數或三位數之黑洞數

Console.Write("x1=");
int n = int.Parse(Console.ReadLine());

int max(string n)
{
    List<char> k = new List<char>(n.ToCharArray());
    k.Sort();
    k.Reverse();

    int res = 0;
    for (int i = 0; i < k.Count; i++)
    {
        int a = (k[i] - '0');
        res += a * (int)Math.Pow(10, k.Count - 1 - i);
    }
    return res;
}

int min(string n)
{
    List<char> k = new List<char>(n.ToCharArray());
    k.Sort();

    int res = 0;
    for (int i = 0; i < k.Count; i++)
    {
        int a = (k[i] - '0');
        res += a * (int)Math.Pow(10, k.Count - 1 - i);
    }
    return res;
}

int k = 2;
while (n != 495 && n != 6174)
{
    if (k != 2) Console.WriteLine();
    int a = max(n.ToString());
    int b = min(n.ToString());
    n = a - b;
    Console.Write($"x{k++}={a}-{b}={n}，");
}
Console.Write("黑洞數=" + n);