//Link: https://codeforces.com/problemset/problem/1536/B

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()
{
	int n; 
	scanf("%d", &n);
	for (int i = 0; i < n; i++)
	{
		int len;
		scanf("%d", &len);
		char input[len+1];
		scanf("%s", input);
		input[len] = '\0';

		int lol = 0;
		char itr[len+1];
		itr[0] = '\0';
		int Z = 0;
		while(lol != 1)
		{
			int j;
			for(j = Z-1; j >= 0; j--)
			{
				if(itr[j] == 'z')
				{
					itr[j] = 'a';
				}
				else
				{
					itr[j] += 1;
					break;
				}
			}
			if(j == -1 || j == -2)
			{
			    for(int k = 0; k <= Z; k++)
			    {
			        itr[k+1] = itr[k];
			    }
			    itr[Z+1] = '\0';
			    Z += 1;
			}
			//itr[Z-1] = 'a';
			for (j = 0; j < 26; j++) 
			{
		    	char ch = 'a' + j;
		    	itr[Z-1] = ch;
		    	char *ret = strstr(input, itr);
		    	if(ret)
		    	{

		    	}
		    	else
		    	{
		    		printf("%s\n", itr);
		    		lol = 1;
		    		break;
		    	}

			}
		}
	}
}