萌新求帮助，不明白怎么错了

#include <iostream>
#include <string>
using namespace std;
void reverse(string &s,int first,int last)
{
char temp;
if(first>=last)
return;
else
{
temp=s[last];
s[last]=s[first];
s[first]=temp;
first++;
last--;
reverse(s,first,last);
}
}
void main()
{
string s="abcdefghijk";
cout<<"字符串正常顺序:"<<s<<endl;
reverse(s,0,s.size());
cout<<" 字符串倒序:"<<s<<endl;
getchar();
}