Longest Palindrome

题意翻译

- 给你 $n$ 个长度为 $m$ 的字符串。 - 请你判断删去其中的几个（或者不删去），能使得将剩下字符串随意排列所形成的回文串长度最大。 - 请你输出最大的长度和那个回文串。 - $1\le n\le 100$，$1\le m\le 50$。 - translate by @[ShineEternal](https://www.luogu.com.cn/user/45475)。

题目描述

Returning back to problem solving, Gildong is now studying about palindromes. He learned that a palindrome is a string that is the same as its reverse. For example, strings "pop", "noon", "x", and "kkkkkk" are palindromes, while strings "moon", "tv", and "abab" are not. An empty string is also a palindrome. Gildong loves this concept so much, so he wants to play with it. He has $ n $ distinct strings of equal length $ m $ . He wants to discard some of the strings (possibly none or all) and reorder the remaining strings so that the concatenation becomes a palindrome. He also wants the palindrome to be as long as possible. Please help him find one.

输入输出格式

输入格式

The first line contains two integers $ n $ and $ m $ ( $ 1 \le n \le 100 $ , $ 1 \le m \le 50 $ ) — the number of strings and the length of each string. Next $ n $ lines contain a string of length $ m $ each, consisting of lowercase Latin letters only. All strings are distinct.

输出格式

In the first line, print the length of the longest palindrome string you made. In the second line, print that palindrome. If there are multiple answers, print any one of them. If the palindrome is empty, print an empty line or don't print this line at all.

输入输出样例

输入样例 #1

3 3
tab
one
bat

输出样例 #1

6
tabbat

输入样例 #2

4 2
oo
ox
xo
xx

输出样例 #2

6
oxxxxo

输入样例 #3

3 5
hello
codef
orces

输出样例 #3

0

输入样例 #4

9 4
abab
baba
abcd
bcde
cdef
defg
wxyz
zyxw
ijji

输出样例 #4

20
ababwxyzijjizyxwbaba

说明

In the first example, "battab" is also a valid answer. In the second example, there can be 4 different valid answers including the sample output. We are not going to provide any hints for what the others are. In the third example, the empty string is the only valid palindrome string.