Sigsegv runtime error

#include
using namespace std;
int main()
{int n,l,i;
char d;
cin>>n;
while(n–)
{ cin>>l;

   string s;
   cin>>s;
   for( i=0;i+1<l;i+=2)
 { d=s[i];
 s[i]=s[i+1];
 s[i+1]=d;
 }
  
     for(i=0;i<l;i++)
     {  s[i]=(char)('z'-s[i]+'a') ;
     }
  cout<<s<<endl;}

return 0;}

can you provide the test case, which cause the SIGSEGV error