大精度求和,给任意两个数 m,n 甚至m,n->∞ 计算x+y

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;


/**
 * 大精度和
 * 
 * 
 *  Scanner sc=new Scanner(System.in);
    BigInteger a=sc.nextBigInteger();
    BigInteger b=sc.nextBigInteger();
    System.out.println(a.add(b));
    sc.close();
 *  可以直接得出答案
 * 
 * 
*@author wenfan
*@version 2018年11月20日上午10:15:59
*/
public class BigSum {
    public static void main(String[] args) throws IOException {
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        String s1=br.readLine();
        String s2=br.readLine();
        String zero="";
        if(s1.length()>s2.length()) {
            for(int i=0;i<s1.length()-s2.length();i++) {
                zero+="0";
            }
            s2=zero+s2;
        }else if(s1.length()<s2.length()) {
            for(int i=0;i<s2.length()-s1.length();i++) {
                zero+="0";
            }
            s1=zero+s1;
        }
        int num1[]=new int[s1.length()+1];
        int num2[]=new int[s2.length()+1];
        for(int i=1;i<=s1.length();i++) {
            num1[i]=Character.getNumericValue(s1.charAt(i-1));
            num2[i]=Character.getNumericValue(s2.charAt(i-1));
        }
        
        int []sum=new int[s1.length()+1];
        
            for(int i=s1.length();i>0;i--) {
                if((num1[i]+num2[i])>10) {
                    sum[i]=sum[i]+(num1[i]+num2[i])%10;
                    sum[i-1]+=1;
                }else if((num1[i]+num2[i])<=9&&sum[i]==0){
                    sum[i]=num1[i]+num2[i];
                }else if((num1[i]+num2[i])<9&&sum[i]==1){
                    sum[i]=num1[i]+num2[i]+sum[i];
                }
                else if((num1[i]+num2[i])==9&&sum[i]==1){
                    sum[i]=0;
                    sum[i-1]+=1;
                }else if((num1[i]+num2[i])==10&&sum[i]==0) {
                    sum[i]=0;
                    sum[i-1]+=1;
                }else if((num1[i]+num2[i])==10&&sum[i]==1) {
                    sum[i]=1;
                    sum[i-1]+=1;
                }
                
            }
             
            if(sum[0]==0) {
                for(int i=1;i<sum.length;i++) {
                    System.out.print(sum[i]);
                }
            }else {
                for(int i=0;i<sum.length;i++) {
                    System.out.print(sum[i]);
                }
            }
        
    }
}    

if语句没有优化，见谅。