10000位内高精度乘法（大数乘法）1…

大整数乘法.模拟小学生算数.

#include <stdio.h>
#include <string.h>
#define size 10000+2
#define truesize size-1-1
int ans[size*size*100]={0};
int multiply(char a[size],int sa,char b[size],int sb){
 int i,j,k,pa,pb,ec=0;
 long last=0;
 for(i=0;(pa=truesize-i)>=sa;i++){
  long cp=0;
  for(j=0;(pb=truesize-j)>=sb;j++){
   int na=a[pa]-'0';
   int nb=b[pb]-'0';
   cp=cp+na*nb;
   ans[ec=truesize-i-j]+=cp;
   cp/=10;   
  }
  while(cp&&ec-1){
   ans[--ec]+=cp;
   cp/=10;
  }
 }
 ec=truesize;
 last=0;
 while(ec>=0){
  last+=ans[ec];
  ans[ec--]=last;
  last/=10;
 }
 for(i=0;i<=truesize;i++)
 if(ans[i])break;
 return i;
}
int read(char c[size]){
 int l=0,i;
 char t[size]="";
 c[truesize+1]='';
 while(t[l]=getchar()){
  if(t[l]=='n'||t[l]==' '){
   l--;
   break;
  }
  l++;
 }
 for(i=0;i<=l;i++)
 c[truesize-(l-i)]=t[i];
 return truesize-l;
}
void show(int i){
 if(i<=truesize)
 for(;i<=truesize;i++)
 printf("%d",ans[i]);
 else
 printf("%d",0);
 printf("n");
}
int main(){
 char a[size]="";
 char b[size]="";
 int sa=read(a);
 int sb=read(b);
 int sc=multiply(a,sa,b,sb);
 show(sc);
 return 0;
}