请设计一个算法能够完成两个用字符串存储的整数进行相加操作，对非法的输入则返回error

 1 #include <iostream>
 2 #include <string>
 3 #include<algorithm>
 4 using namespace std;
 5 int main(){
 6     string str1;
 7     string str2;
 8     while(cin>>str1>>str2){
 9         int size1 = str1.size()-1;
10         int size2 = str2.size()-1;
11         int flag = 0;
12         for(int i=0;i<=size1;i++){
13             if(!(str1[i]>=‘0‘ && str1[i] <= ‘9‘)){
14                 cout<<"error"<<endl;
15                 flag = 1;
16                 break;
17             }
18         }
19         if(flag) break;
20         for(int i=0;i<=size2;i++){
21              if(!(str2[i]>=‘0‘ && str2[i] <= ‘9‘)){
22                 cout<<"error"<<endl;
23                  flag = 1;
24                 break;
25             }
26         }
27         if(flag) break;
28         string sum;
29         int carry = 0;
30         int j = 0;
31         while(size1>=0 && size2>=0){
32             int nsum = (str1[size1] - ‘0‘) + (str2[size2] - ‘0‘) + carry;
33             carry = nsum/10;
34             nsum = nsum%10;
35             sum += to_string(nsum);
36             size1--;
37             size2--;
38         }
39         while(size1 >= 0){
40             int nsum = (str1[size1] - ‘0‘) + carry;
41             carry = nsum/10;
42             nsum = nsum%10;
43             sum += to_string(nsum);
44             size1--;
45         }
46         while(size2 >= 0){
47             int nsum = (str2[size2] - ‘0‘) + carry;
48             carry = nsum/10;
49             nsum = nsum%10;
50             sum += to_string(nsum);
51             size2--;
52         }
53         if(carry){
54             sum += ‘1‘;
55         }
56         reverse(sum.begin(),sum.end());
57         cout<<sum<<endl;
58     }
59 }