poj 2389 Bull Math

二つの大きな数を掛け合わせる
 

#include <stdio.h>
#include<string.h>

const int MAXN=100000;
char ch1[MAXN],ch2[MAXN];
int sum[MAXN];

int main(int argc, char *argv[])
{
	int i,j,Rem;
	char t;
	memset(ch1,0,sizeof(ch1));
	memset(ch2,0,sizeof(ch2));
	scanf("%s",ch1);
	scanf("%s",ch2);
	memset(sum,0,sizeof(sum));
	int len1=strlen(ch1);
	int len2=strlen(ch2);
	for(i=0;i<len1;i++)
	  ch1[i]-=48;
	for (i=0;i<len2;i++)
	  ch2[i]-=48;  
	for (i=0;i<len1/2;i++)//   
    {
  		 t=ch1[i];
  		 ch1[i]=ch1[len1-1-i];
  		 ch1[len1-1-i]=t;
  	}
	for (i=0;i<len2/2;i++)//   
    {
  		 t=ch2[i];
  		 ch2[i]=ch2[len2-1-i];
  		 ch2[len2-1-i]=t;
  	}
  	int cas=0;
    for (i=0;i<len1;i++)
    {
	     int c=0;
		for (j=0;j<MAXN;j++)
		{
			int s=ch1[i]*ch2[j]+c;
			sum[j+cas]=sum[j+cas]+s%10;
			c=s/10;
		}
		cas++;	
	}
	int c=0;
	for (i=0;i<MAXN;i++)
	{/*
	    Rem=sum[i];
	    sum[i]=(sum[i]+c)%10;
		c=Rem/10; 
	r-1 +c      r          (19+c>20  c    1   2)	
	*/	
		sum[i]=sum[i]+c;
		Rem=sum[i];
		sum[i]%=10;
		c=Rem/10;
	}
	for (i=MAXN-1;i>=0;i--)
	   if(sum[i]) break;
    for (;i>=0;i--)
	   printf("%d",sum[i]);
    printf("
");
	return 0;
}