两个超大数的加法-遇到的一个笔试题

最近碰到的一个笔试题，应该有多种实现方式，我也只是那一个最常用的方法搞定的

import java.math.BigInteger;
import java.util.regex.Pattern;

public class AddBigData {

    private static final int SINGLE_MAX = 9;
    private static final int ONE = 1;

    public static String addBigData(String fir, String sec) throws NumberFormatException{
        //check format is ok, first to check weather exist zero
        if("0".equals(fir)){
            return sec;
        }else if("0".equals(sec)){
            return fir;
        }else {
            //use regx to check number format
            Pattern dataPattern = Pattern.compile("^[1-9]\d*$");//start not with zero ,end with data
            boolean firCheck = dataPattern.matcher(fir).find();
            boolean secCheck = dataPattern.matcher(sec).find();
            
            if(firCheck && secCheck){
                //begin to cal
                return add(fir, sec);
            }else{
                //throw exception
                throw new NumberFormatException("输入的字符串格式错误！请输入正确的数字。");
            }
        }
    }
    
    public static String add(String fir, String sec) {
        StringBuilder res = new StringBuilder();
        
        char[] firs = fir.toCharArray();
        char[] secs = sec.toCharArray();
        //get min length to cal, get maxlen to store
        int minLen = Math.min(firs.length, secs.length);
        int maxLen = Math.max(firs.length, secs.length);
        
        int[] aiRes = new int[maxLen+1];//to store, be carefull the aiRes's length must be maxLen+1
        //init aiRes
        aiRes[0] = 0;
        char[] maxString = firs.length >= secs.length ? firs : secs;
        for(int i = 0; i < (maxLen - minLen); i++){
            aiRes[i+1] = Integer.parseInt(String.valueOf(maxString[i]));
        }
        
        int prompt = 0;//data to add in the next step
        //begin to cal
        for(int i = 0; i < minLen; i++){
            char cFir = firs[firs.length - 1 - i];
            char cSec = secs[secs.length - 1 - i];
            //change to number and add between (max-min) to max-1
            int iFir = Integer.parseInt(String.valueOf(cFir));
            int iSec = Integer.parseInt(String.valueOf(cSec));
            
            iFir += iSec;
            iFir += prompt;
            aiRes[aiRes.length - 1 - i] = iFir % 10;
            prompt = iFir / 10;
        }
        aiRes[maxLen - minLen] += prompt;
        //is needed to cal continue
        if(aiRes[maxLen - minLen] > SINGLE_MAX){
            aiRes[maxLen - minLen] %= 10;
            //cal the last of aiRes between 0 to max-min-1
            for(int i = maxLen - minLen - 1; i >= 0; i--){
                aiRes[i] += ONE;
                if (aiRes[i] > SINGLE_MAX) {
                    aiRes[i] %= 10;
                }else{
                    break;
                }
            }
        }
        
        if(aiRes[0] > 0){
            res.append(aiRes[0]);
        }
        for(int i = 1; i < aiRes.length; i++){
            res.append(aiRes[i]);
        }
        
        return res.toString();
    }

    /**
     * @param args
     */
    public static void main(String[] args) {
        String firstBigData = "19911999998888888888888888888888888888888888888888999999999999999999999999999999";
        String secBigData =    "1008000001111111111111111111111111111111111111111000000000000000000000000000001";
        System.out.println(addBigData(firstBigData, secBigData));
        BigInteger bi1 = new BigInteger(firstBigData);
        BigInteger bi2 = new BigInteger(secBigData);
        bi1 = bi1.add(bi2);
        System.out.println(bi1.toString());
        /*result: 20920000000000000000000000000000000000000000000000000000000000000000000000000000*/
    }

}