咱就是说这个代码哪里错了？？？？

# include <iostream>
# include <string>
# include <list>
using namespace std;
int main()
{
    string a , s;
    cin >> a >> s;
    list<char> alist , blist;
    for(int i = 0 ; i < a.size() ; i ++) alist.push_back(a[i]);
    for(int i = 0 ; i < s.size() ; i ++) blist.push_back(s[i]);
    if(alist.size() < blist.size())
    {
        while(alist.size() != blist.size())
        {
            alist.push_front(' ');
        }
    }
    else if (alist.size() > blist.size())
    {
        while(alist.size() != blist.size())
        {
            blist.push_front(' ');
        }
    }
    string ans1 , ans2;
    while(!alist.empty())
    {
        ans1 += alist.front() ;
        alist.pop_front();
    }
    while(!blist.empty())
    {
        ans2 += blist.front() ;
        blist.pop_front();
    }
    string ans;
    for(int i = 0 ; i < ans1.size() ; i ++) ans += ans1[i] - '0' + ans2[i] - '0';
    cout << ans;
}

我用string拿进来，将全部的字符全存在list里面，（在位数不等的情况下加‘0’），然后再拿到string里，最后将结果加起来输出？？？