窝咋对了？

#include<bits/stdc++.h>
using namespace std;
string m(string a,string b)
{
	if(a=="0"||b=="0") return "0";
	reverse(a.begin(),a.end());
    reverse(b.begin(),b.end());
    vector<int> r(a.size()+b.size(),0);
    for (int i = 0; i < a.size(); i++)
	{
        for (int j = 0; j < b.size(); j++)
		{
            r[i+j] += (a[i] - '0') * (b[j] - '0');
            r[i+j+1] += r[i + j] / 10;
            r[i+j] %= 10;
        }
    }
    while(r.size()>1&&r.back()==0) r.pop_back();
    string ans;
    for (int i=r.size()-1; i >= 0; --i)
	{
        ans += to_string(r[i]);
    }
    return ans;
}
int main()
{
	cin>>a>>b;
	cout<<m(a,b)<<endl;
	return 0;
}