1688C · Manipulating History

Problem: Keine has the ability to manipulate history.

The history of Gensokyo is a string $s$ of length $1$ initially. To fix the chaos caused by Yukari, she needs to do the following operations $n$ times, for the $i$-th time:

• She chooses a non-empty substring $t_{2i-1}$ of $s$.
• She replaces $t_{2i-1}$ with a non-empty string, $t_{2i}$. Note that the lengths of strings $t_{2i-1}$ and $t_{2i}$ can be different.

Note that if $t_{2i-1}$ occurs more than once in $s$, exactly one of them will be replaced.

For example, let $s=$"marisa", $t_{2i-1}=$"a", and $t_{2i}=$"z". After the operation, $s$ becomes "mzrisa" or "marisz".

After $n$ operations, Keine got the final string and an operation sequence $t$ of length $2n$. Just as Keine thinks she has finished, Yukari appears again and shuffles the order of $t$. Worse still, Keine forgets the initial history.

Help Keine find the initial history of Gensokyo!

Recall that a substring is a sequence of consecutive characters of the string. For example, for string "abc" its substrings are: "ab", "c", "bc" and some others. But the following strings are not its substring: "ac", "cba", "acb".

Hacks

You cannot make hacks in this problem.

Input Format: Each test contains multiple test cases. The first line contains a single integer $T$ ($1 \leq T \leq 10^3$) — the number of test cases. The description of the test cases follows.

The first line of each test case contains a single integer $n$ ($1 \le n < 10 ^ 5$) — the number of operations.

The next $2n$ lines contains one non-empty string $t_{i}$ — the $i$-th string of the shuffled sequence $t$.

The next line contains one non-empty string $s$ — the final string.

It is guaranteed that the total length of given strings (including $t_i$ and $s$) over all test cases does not exceed $2 \cdot 10 ^ 5$. All given strings consist of lowercase English letters only.

It is guaranteed that the initial string exists. It can be shown that the initial string is unique.

Output Format: For each test case, print the initial string in one line.

Note: Test case 1:

Initially $s$ is "a".

• In the first operation, Keine chooses "a", and replaces it with "ab". $s$ becomes "ab".
• In the second operation, Keine chooses "b", and replaces it with "cd". $s$ becomes "acd".

So the final string is "acd", and $t=[$"a", "ab", "b", "cd"$]$ before being shuffled.

Test case 2:

Initially $s$ is "z".

• In the first operation, Keine chooses "z", and replaces it with "aa". $s$ becomes "aa".
• In the second operation, Keine chooses "a", and replaces it with "ran". $s$ becomes "aran".
• In the third operation, Keine chooses "a", and replaces it with "yakumo". $s$ becomes "yakumoran".

So the final string is "yakumoran", and $t=[$"z", "aa", "a", "ran", "a", "yakumo"$]$ before being shuffled.