Finding the factorial algorithm of large numbers

Factorial very good, recursive on the line, but for the larger number, factorial results very very large, wood has a way, can only be handled by string.

We use string multiplication to do it. After testing, this program can run a maximum of n value of almost 3000, and Windows on the scientific calculator of the same:)

String multiply (string num1, String num2) {int len1 = Num1.size (), len2 = Num2.size (), Len = Len1 + len2;        String str (len, ' 0 ');            for (int i = len1-1; I >= 0; i--) {int a = Num1[i]-' 0 ';                for (int j = len2-1, k = Len2 + i; j >= 0; J--, k--) {int b = num2[j]-' 0 ';                int c = str[k]-' 0 ';                int t = b * a + C;                STR[K] = t% 10 + ' 0 ';                int d = (str[k-1]-' 0 ') + T/10;                if (d >=) str[k-2] = str[k-2]-' 0 ' + D/10 + ' 0 ';            STR[K-1] = d% 10 + ' 0 ';        }} int x = 0;        while (str[x] = = ' 0 ') x + +;        if (Str.substr (x, len-x) = = "") return "0";    return Str.substr (x, len-x);    }string itoa (int n) {string s= "";        do{s+=n%10+ ' 0 ';    n/=10;    }while (n);    Reverse (S.begin (), S.end ()); return s;} string fun (int n) {string s="1";    for (int i=2;i<=n;i++) {s=multiply (S,itoa (i)); } return s;}

Finding the factorial algorithm of large numbers