自己写的。大数加法。字符串转int数组。

import java.util.*;
import java.lang.*;


public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        while (in.hasNextLine()) {
            String str1 = in.nextLine();
            String str2 = in.nextLine();
            StringBuffer sb = new StringBuffer(str1);
            str1 = sb.reverse().toString();  //反转加数a
            sb = new StringBuffer(str2);
            str2 = sb.reverse().toString(); //反转加数b
            int n = Math.max(str1.length(), str2.length()) + 1;  //长度+1，为最高位有进位做准备
            int[] a = new int[n];
            int[] b = new int[n];
            int i = 0;
            for (char ch : str1.toCharArray()) {
                a[i++] = ch - '0';        //string转int
            }
            for (i = str1.length(); i < n; i++) {
                a[i] = 0;                 //最高位前面补零
            }
            i = 0;
            for (char ch : str2.toCharArray()) {
                b[i++] = ch - '0';
            }
            for (i = str2.length(); i < n; i++) {
                b[i] = 0;
            }
            int add = 0;  //进位
            for (i = 0; i < n; i++) {
                int sum = a[i] + b[i] + add;   
                add = sum / 10;
                a[i] = sum % 10;    //将结果保存在a中
            }
            String res = "";
            for (i = 0; i < n; i++) {
                if (i == n - 1 && a[i] == 0) {   //如果最高位没有进位，则之前补的0要舍弃。
                    break;       
                }
                res += String.valueOf(a[i]);    //将答案依次附加到结果字符串中
            }
            sb = new StringBuffer(res);        //反转得到正确的结果
            System.out.println(sb.reverse().toString());
        }
    }
}