Add two decimal number stored in strings

add.cpp

string add(const string & s1, const string & s2)
{
    int flag = 0;
    string ret(std::max(s1.size(),s2.size())+1, '0');
    for (int i = 0; i < ret.size(); i++) {
        int v = (i < s1.size() ? s1[s1.size()-1-i]-'0' : 0)
              + (i < s2.size() ? s2[s2.size()-1-i]-'0' : 0) + flag;
        flag = v/10;
        v -= flag*10;
        ret[ret.size()-1-i] = v+'0';
    }
        
    while (ret[0] == '0')
        ret.erase(ret.begin());
        
    return ret;
}