Why is it showing compilation error

#1

import java.util.*;
public class Main {
public static void main(String args[]) {
Scanner scan=new Scanner(System.in);
while(scan.hasNextInt()){
int tc=scan.nextInt();
for(int i=1;i<tc+1;i++){
int n=scan.nextInt();
classAssignment(n,"");
System.out.println("#"+i+" : "+count);
count=0;
}
}
}
public static void classAssignment(int n,String ans){
if(ans.length()==n){
count++;
return;
}
classAssignment(n,ans+“a”);
if(ans.length()==0 || ans.charAt(ans.length()-1)!=‘b’){
classAssignment(n,ans+“b”);
}
}
}

#2

it is showing run error for one of the test case