Product //大数相乘-蒲公英云

Product //大数相乘

#include <stdio.h>
#include <string.h>
#define N 1000
void yiwei(char *ch, int a);
void add(char *ch1, char *ch2);
void rev(char *ch);
int main()
{
    char ch1[N], ch2[N], ch3[N], ch4[N];
    int i, j, k;
    while(scanf("%s %s", ch1, ch2) != -1)
    {
        rev(ch1);rev(ch2);
        memset(ch4, 0, sizeof(ch4));
        for( i=0; ch1[i]!=0; i++)
        {
            memset(ch3, 0, sizeof(ch3));
            for( j=0; ch2[j]!=0; j++)       //ch3中存的是对应位乘积+1，不加1可能为0了。
            {
                ch3[j] =  (ch1[i]-'0')*(ch2[j]-'0')+1;
            }
            for( k=0; ch3[k]; k++)
            {
                if(ch3[k]-1 > 9)
                {
                    if(!ch3[k+1])
                    {
                        ch3[k+1] += (ch3[k]-1)/10+1;
                        ch3[k] = (ch3[k]-1)%10+1;
                    }
                    else
                    {
                        ch3[k+1] += (ch3[k]-1)/10;
                        ch3[k] = (ch3[k]-1)%10+1;
                    }
                }
           } 
            k = 0;
             while(ch3[k])
             {    
                 ch3[k] = ch3[k]-1+'0';    
                 k++;
             }
            yiwei(ch3, i);
            add(ch4, ch3);
        }
        rev(ch4);
        if(ch4[0]=='0')
            ch4[1] = 0;
        printf("%s\n", ch4);
    }
    return 0;
}
void yiwei(char *ch, int a)
{
    int len, i;
    len = strlen(ch);
    for( i=len+a; i>=a; i--)
    {
        ch[i] = ch[i-a];
    }
    for(i=0; i<a; i++)
        ch[i] = '0';
}
void add(char *ch1, char *ch2)
{
    int i = 0;
    while(ch1[i] || ch2[i])
    {
        if(!ch1[i] || !ch2[i])
            ch1[i] = ch1[i]+ch2[i];
        else
            ch1[i] = ch1[i]+ch2[i]-'0';
        i++;
    }
    i = 0;
    while(ch1[i])
    {
        if(ch1[i] > '9')
        {
            if(!ch1[i+1])
            {
                ch1[i+1] += (ch1[i]-'0')/10+'0';
                ch1[i] = '0'+(ch1[i]-'0')%10;
            }
            else
            {
                ch1[i+1] += (ch1[i]-'0')/10;
                ch1[i] = '0'+(ch1[i]-'0')%10;
            }
        }
        i++;
    }
}
void rev(char *ch)
{
    int len, i;
    char ch1[N];
    len = strlen(ch);
    for(i=0; i<len; i++)
        ch1[len-i-1] = ch[i];
    for(i=0; i<len; i++)
        ch[i] = ch1[i];
}