A + B problem 高精度

#include <bits/stdc++.h>
using namespace std;
int main()
{
    string a,b;
    cin>>a>>b;
    int i1 = a.size() - 1;
    int i2 = b.size() - 1;
    string s;
    int carry = 0;
    while (i1 >= 0 || i2 >= 0)
    {
        char ch = carry;
        if (i1 >= 0)
        {
            if (a[i1] < ‘0‘ || a[i1] > ‘9‘)
                continue;
            ch += a[i1] - ‘0‘;
        }
        if (i2 >= 0)
        {
            if (b[i2] < ‘0‘ || b[i2] > ‘9‘)
                continue;
            ch += b[i2] - ‘0‘;
        }
        if (ch >= 10)
        {
            carry = 1;
            ch -= 10;
        }
        else carry = 0;
        s.push_back(ch + ‘0‘);
        i1--;
        i2--;
    }
    if (carry)
    s.push_back(‘1‘);
    reverse(s.begin(), s.end());
    return s;
}