#include<iostream>
#include<cstring>
#include<string>
#include<cstdio>
#include<cmath>
#include<algorithm>
using namespace std;
int a[2005]={};
int b[2005]={};
int c[4010]={};
int res[4010];
string s1,s2; 
int main(){
	cin >> s1 >> s2;
	int lena = s1.length(); 
	int lenb = s2.length(); 
	int len = lena+lenb;
	for(int i=lena-1,j=1;i>=0;i--,j++){
		a[j] = s1[i] - '0';
	}
	for(int i=lenb-1,j=1;i>=0;i--,j++){
		b[j] = s2[i] - '0';
	}
	for(int i=1;i<=lena;i++){
		for(int j=1;j<=lenb;j++) c[j+i-1] += a[i]*b[j];
	}
	int tmp=0;
	for(int i=1;i<=len;i++){
		c[i+1] += c[i]/10;
		c[i]%=10;
	}
	while(c[len]==0&&len !=1){
		len--;
	}
	for(int i=len;i>=1;i--){
		cout << c[i];
	}
	return 0;
}

高精度

#include <iostream>
#include <cstring>
#define max 1000000
using namespace std;
int main()
{
	int a[max],b[max],c[max+1];
	string sa,sb;
	memset(a,0,sizeof(a));
	memset(b,0,sizeof(b));
	memset(c,0,sizeof(c));
	int x = 0;
	cin>>sa>>sb;
	if (sa.size() >= max)
	{
		return 0;
	}
	if (sb.size() >= max)
	{
		return 0;
	}
	int lena = sa.size();
	int lenb = sb.size();
	for (int i = 0;i < sa.size();i++)
	{
		a[lena-i] = sa[i] - '0';
	}
	for (int i = 0;i < sb.size();i++)
	{
		b[lenb-i] = sb[i] - '0';
	}
	int lenc = 0;
	for (int i = 1;i <= lena;i++)
	{
		x = 0;
		for (int j = 1;j <= lenb;j++)
		{
			c[i+j-1] = a[i]*b[j]+x+c[i+j-1];
			x = c[i+j-1] / 10;
			c[i+j-1] %= 10;
		}
		c[i+lenb] = x;
	}
	lenc=lena+lenb;
	for (int i = 0;i <= lenc;i++)
	{
		if (c[lenc] == 0)
		{
			lenc--;
		}
		if (c[lenc] > 0)
		{
			break;
		}
	}
	for (int i = lenc;i >= 1;i--)
	{
		cout<<c[i];
	}
	return 0;
}