小明的阶乘

#include<iostream>
#include<algorithm>
#include<cstring>
using namespace std;
char a[10000],b[10000],result[10000];
void call()
{
    int i,j,ex,res,lena,lenb,len,r;
    lena=strlen(a);
    lenb=strlen(b);
    len=lena+lenb;
    for(i=0;i<=len;i++)
        result[i]='0';
    reverse(a,a+lena);
    reverse(b,b+lenb);
    r=-1;
    for(i=0;i<lena;i++)
    {
        ex=0;
        for(j=0;j<lenb;j++)
        {
            res=(a[i]-'0')*(b[j]-'0')+ex+result[i+j]-'0';
            result[i+j]=res%10+'0';
            ex=res/10;
            if(i+j>r)r=i+j;
        }
        while(ex!=0)
        {
            res=result[i+j]-'0'+ex;
            result[i+j]=res%10+'0';