Tanya and Password

题意翻译

当爸爸在工作时，一个小女孩Tanya决定玩爸爸的密码进入他的秘密数据库。爸爸的密码是一个由n+2个字符组成的字符串。她把密码的所有可能的n三个字母的连续子串都写在一张纸上，每一张纸一个，然后把密码扔了出去。每个三个字母的子串被写入密码的次数。因此，Tanya最终得到了n张纸。 然后Tanya意识到爸爸在得知她的游戏后会很不高兴，于是决定恢复密码，或者至少恢复与最后一组三个字母字符串相对应的任何字符串。你必须帮助她完成这项艰巨的任务。我们知道爸爸的密码由拉丁字母和数字的小写字母和大写字母组成。拉丁字母表中的大写字母和小写字母被认为是不同的。

题目描述

While dad was at work, a little girl Tanya decided to play with dad's password to his secret database. Dad's password is a string consisting of $ n+2 $ characters. She has written all the possible $ n $ three-letter continuous substrings of the password on pieces of paper, one for each piece of paper, and threw the password out. Each three-letter substring was written the number of times it occurred in the password. Thus, Tanya ended up with $ n $ pieces of paper. Then Tanya realized that dad will be upset to learn about her game and decided to restore the password or at least any string corresponding to the final set of three-letter strings. You have to help her in this difficult task. We know that dad's password consisted of lowercase and uppercase letters of the Latin alphabet and digits. Uppercase and lowercase letters of the Latin alphabet are considered distinct.

输入输出格式

输入格式

The first line contains integer $ n $ ( $ 1<=n<=2·10^{5} $ ), the number of three-letter substrings Tanya got. Next $ n $ lines contain three letters each, forming the substring of dad's password. Each character in the input is a lowercase or uppercase Latin letter or a digit.

输出格式

If Tanya made a mistake somewhere during the game and the strings that correspond to the given set of substrings don't exist, print "NO". If it is possible to restore the string that corresponds to given set of substrings, print "YES", and then print any suitable password option.

输入输出样例

输入样例 #1

5
aca
aba
aba
cab
bac

输出样例 #1

YES
abacaba

输入样例 #2

4
abc
bCb
cb1
b13

输出样例 #2

NO

输入样例 #3

7
aaa
aaa
aaa
aaa
aaa
aaa
aaa

输出样例 #3

YES
aaaaaaaaa