/* @JUDGE_ID:4461XX 10701 C */
/* A */
#include<stdio.h>
#include<string.h>

int stringLen;
char string1[80], string2[80];
char *root;

void findPost( char *in )
{
	char rootChar;
	char *index;
	
	if( in==NULL )
		return;
	for( rootChar=*root,index=in; *index; ++index )
		if( *index==rootChar ){
			*index = NULL;
			++root;

			findPost( in );
			findPost( index+1 );
			putchar( rootChar );
			break;
		}
}
int main( void )
{
	int cases;

	scanf( "%d\n", &cases );
	for( ; cases; --cases ){
		scanf( "%d %s %s\n", &stringLen, string1, string2 );

		root = string1;
		findPost( string2 );
		putchar( '\n' );
	}

	return 0;
}