两个无符号的正大数相加

public static String bigNumAdd(String s1,String s2){
        if(s1==null||"".equals(s1)){
            return s2;
        }
        if(s2==null||"".equals(s2)){
            return s1;

        }
        char[] sArr1 = new StringBuffer(s1).reverse().toString().toCharArray();
        char[] sArr2 = new StringBuffer(s2).reverse().toString().toCharArray();
        int maxLen = sArr1.length>sArr2.length?sArr1.length:sArr2.length;
        int[] res = new int[maxLen+1];
        for(int i=0;i<=maxLen;i++){
            int i1 = i<sArr1.length?sArr1[i] - ‘0‘:0;
            int i2 = i<sArr2.length?sArr2[i] - ‘0‘:0;
            int tmp = i1+i2;
            if(tmp>=10){
                res[i+1] += 1;
                res[i] += tmp -10;
            }else{
                res[i] += tmp;
            }
        }
        StringBuffer sb = new StringBuffer();
        if(res[res.length-1]!=0){
            sb.append(res[res.length-1]);
        }
        for(int i=res.length-2;i>=0;i-- ){
            sb.append(res[i]);
        }
        return sb.toString();

    }