高精度乘法

import java.math.BigInteger;
import java.util.*;
public class Main{
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        String s1=sc.next();
        String s2=sc.next();
        int res[]=new int[s1.length()+s2.length()];
        for(int i=s1.length()-1;i>=0;i--){
            int x=s1.charAt(i)-'0';
            for(int j=s2.length()-1;j>=0;j--){
                int y=s2.charAt(j)-'0';
                int s=x*y+res[i+j+1];
                res[i+j]+=s/10;
                res[i+j+1]=s%10;
            }
        }
        StringBuilder sb=new StringBuilder();
        int k=0;
        while(k<res.length&&res[k]==0) k++;
        if(k==res.length) System.out.println(0);
        else{
            while(k<res.length) sb.append(res[k++]);
            System.out.println(sb.toString());
        }
    }
}

BigInteger

import java.math.BigInteger;
import java.util.*;
public class Main{
    public static void main(String[] args) {
        Scanner sc=new Scanner(System.in);
        String s1=sc.next();
        String s2=sc.next();
        BigInteger a=new BigInteger(s1);
        BigInteger b=new BigInteger(s2);
        System.out.println(a.multiply(b).toString());
    }
}