2. 有趣的數列

string k = "1";
Console.Write("N=");
int t = int.Parse(Console.ReadLine());
Console.WriteLine(k);
for (int i = 0; i < t; i++)
{
    string str = "";
    char? last = null;
    int n = 0;
    for (global::System.Int32 j = 0; j < k.Length; j++)
    {
        var c = k[j];
        if (last == null)
        {
            n = 1;
            last = c;
        }
        else if(c == last)
        {
            n++;
        }
        else
        {
            str += $"{n}{last}";
            last = c;
            n = 1;
        }
    }
    str += $"{n}{last}";
    k = str;
    Console.WriteLine(k);
}