char str[200][100] ;//Maximum calculation 200! The number is up to 100 bits

In str[0][]=1;

str[1][]=1;

str[2] []=2;

str[3][]=str[2][]*3;

str[4][]=str[3][]*4;..... This way the calculation will be fast;

Then all you have to do is write your own big data multiplication, (String A and String B simulate multiplying two numbers, if you can't write it you search online for big data multiplication C)