UVA 10106 Product （大数相乘）

Product 

The Problem

The problem is to multiply two integers X, Y. (0<=X,Y<10²⁵⁰)

The Input

The input will consist of a set of pairs of lines. Each line in pair contains one multiplyer.

The Output

For each input pair of lines the output line should consist one integer the product.

Sample Input

12
12
2
222222222222222222222222

Sample Output

144
444444444444444444444444

大数相乘，网上有非常多模板，今下午练练手，自己写一遍，刚開始忽略了前置0。导致0*12=00,0*123=000，后来改了输出就ac了。

#include <iostream>
#include <cstring>
#define maxn 260
using namespace std;
int main()
{
    char str1[maxn],str2[maxn];
    while(cin>>str1>>str2)
	{		
		int len1,len2;
		len1=strlen(str1);
		len2=strlen(str2);
		
		int s1[maxn],s2[maxn];
		memset(s1,0,sizeof(s1));
		memset(s2,0,sizeof(s2));
		
		int i;
		for(i=0;i<len1;i++)
			s1[i]=str1[i]-'0';
		for(i=0;i<len2;i++)
			s2[i]=str2[i]-'0';
		
		int sum[maxn+maxn];
		memset(sum,0,sizeof(sum));
		int j,k;
		for(i=len2-1;i>=0;i--)
		{
			for(j=len1-1;j>=0;j--)
			{
				sum[i+j+1]=sum[i+j+1]+s2[i]*s1[j];
			}
		}
		for(k=len1+len2-1;k>=0;k--)
			if(sum[k]>=10)
			{
				int temp;
				temp=sum[k]%10;
				sum[k-1]=sum[k-1]+sum[k]/10;
				sum[k]=temp;
			}
			
			//if(sum[0]!=0)
			//	cout<<sum[0];
			bool flag=0;
			for(i=0;i<len1+len2-1;i++)
			{
				if(sum[i]!=0)
					flag=1;
				if(flag==1)
				cout<<sum[i];
			}
			cout<<sum[len1+len2-1];
			cout<<endl;
	}
    return 0;
}