rotation of string

you are given two string s1 and s2 and you have to tell whether string s2 is rotation of string s1 or not you have to use strstr function only once ?
example s1=abcde
s2=cdeab
here s2 is a rotation of s1

s1=s1+s1;
strstr(s1,s2) will tell whether rotation or not

#include<iostream>
#include<string>
#include<conio.h>
using namespace std;
int main()
{
char s1[100],s2[100],s[100];
cin>>s1;
cin>>s2;
strcpy(s,s1);
strcat(s1,s);
if(strstr(s1,s2)!=NULL)
cout<<"rotation";
else
cout<<"no rotation";
getch();
}
logic of this programm is that if we add any string two times it will contain all of its rotation.