first-non-repeating-character-in-a-stream

import java.util.*;
import java.lang.*;
import java.io.*;
class GFG
{
	public static void main (String[] args) throws Exception
	{ 
	    BufferedReader bf = new BufferedReader(new InputStreamReader(System.in));
	    int t = Integer.parseInt(bf.readLine());
	    while(t-- > 0)
	    {
	        int n = Integer.parseInt(bf.readLine());
	        String s = bf.readLine().trim();
	        int freq[] = new int[26];
	        
	        Queue<Character> q = new LinkedList<Character>();
	        StringBuffer sb = new StringBuffer();
	        
	        for(int i=0 ; i<n ; i++)
	        {
	            char ch = s.charAt(i*2);
	            freq[ch - 'a']++;
	            q.add(ch);
	            
	            while(!q.isEmpty())
	            {
	                if(freq[q.peek() - 'a'] > 1)
	                    q.poll();
	                else
	                {
	                    sb.append(q.peek() + " ");
	                    break;
	                }
	            }
	            
	            if(q.isEmpty())
	                sb.append(-1 + " ");
	            
	        }
	        System.out.println(sb);
	    }
	}
}