超长数字相乘/大数相乘（java）

不难但是第一次见的话写起来很麻烦

作业帮图像算法一面原题（白板）

import java.util.Scanner;
class test{
    public static void main(String[] args){
        Scanner in = new Scanner(System.in);
        while (in.hasNext()){
            String s1 = in.nextLine();
            String s2 = in.nextLine();
            String res = "";
            for (int i = s1.length()-1; i >= 0; i--){
                String cur = mult(s2, s1.charAt(i));
                int k = s1.length() - 1 - i;
                while (k > 0){
                    cur += "0";
                    k--;
                }
                res = add(res, cur);
            }
            System.out.println(res);
        }
    }
    private static String add(String s1, String s2){
        StringBuilder sb = new StringBuilder();
        if (s1.length() < s2.length()){
            int k = s2.length() - s1.length();
            while (k > 0){
                s1 = "0" + s1;
                k--;
            }
        }else{
            int k = s1.length() - s2.length();
            while (k > 0){
                s2 = "0" + s2;
                k--;
            }
        }
        int jw = 0;
        for (int i = s1.length()-1; i >= 0; i--){
            int cur = (int)(s1.charAt(i) + s2.charAt(i) + jw - '0' - '0');
            jw = cur / 10;
            cur %= 10;
            sb.append(cur);
        }
        if (jw == 1) sb.append(1);
        return sb.reverse().toString();
    }
    private static String mult(String s1, char s2){
        StringBuilder sb = new StringBuilder();
        int jw = 0;
        for (int i = s1.length()-1; i >= 0; i--){
            int cur = (int)(s1.charAt(i) - '0') * (int)(s2 - '0') + jw;
            jw = cur / 10;
            cur = cur % 10;
            sb.append(cur);
        }
        if (jw != 0) sb.append(jw);
        return sb.reverse().toString();
    }
}