/*
 * tree03.cpp
 *
 *  Created on: Apr 12, 2024
 *      Author: 28032
 */


//建树
/*#include <iostream>
#include <string>
#include <set>
#include <algorithm>

using namespace std;

struct Note
{
	Note(int val)
	{
		value = val;
		left = NULL;
		right = NULL;
	}

	char value;
	Note* left;
	Note* right;
};
string front,middle;
set<char> exist;
Note* reBuildTree(unsigned int f_index,int m_l,int m_r)
{
	char rootValue = front[f_index];
	while(exist.count(rootValue)&&f_index<front.size())
		rootValue = front[++f_index];

	Note* root = new Note(rootValue);
	exist.insert(rootValue);

	int m_index = middle.find(rootValue);
	string L = middle.substr(m_l,m_index-m_l);
	if(L.size() > 1)
		root->left = reBuildTree(f_index+1,m_l,m_index-1);
	else if(L.size() == 1)
	{
		exist.insert(L[0]);
		root->left = new Note(L[0]);
	}
	else
		root->left = NULL;

	string R = middle.substr(m_index+1,m_r-m_index);
	if(R.size() > 1)
		root->right = reBuildTree(f_index+1,m_index+1,m_r);
	else if(R.size() == 1)
	{
		exist.insert(R[0]);
		root->right = new Note(R[0]);
	}
	else
		root->right = NULL;


	return root;
}

void backVisit(Note* root)
{
	if(root == NULL)	return ;
	backVisit(root->left);
	backVisit(root->right);

	cout<<root->value;
}


int main()
{
	while(1)
	{
		int n = 0;
		cin>>n;
		if(!n)	break;
		cin>>front>>middle;
		exist.clear();
		Note* root = reBuildTree(0,0,n-1);
		backVisit(root);
		cout<<endl;
	}


	return 0;
}*/