A. Word Ladder
Ограничения: время – 2s/4s, память – 64MiB Ввод: input.txt или стандартный ввод Вывод: output.txt или стандартный вывод 
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A word ladder is a sequence of words, in which two consecutive words differ by exactly
one letter. An example of such a ladder (usually arranged vertically, hence the "ladder")
would be: beer, brew, brow, word, down. Note that to get from one word to the next, the
letters may be rearranged, and exactly one letter is changed.
For this problem, you will be given a dictionary of distinct words, all of the same length.
Your task is to write a program that finds a word ladder of minimal length, such that the
first and last word of the ladder have no letters in common.
Input
On the first line an integer t (1 ): the number of test cases. Then for each test
case:
- A line with two space-separated integers n (2\ ≤\ n\ ≤\ 100) and l (1\ ≤\ l\ ≤\ 20): the number of words and their length.
- n lines with a word, each consisting of l lowercase letters (a – z).
Output
For each testcase:
- a single line with the words in a ladder of minimal length, separated by a single space.
It is guaranteed that at least one such ladder can be constructed. If there is more than
one, output the one that comes first lexicographically.
Notes
If s and t are strings of equal length and s_i denotes the ith character of s, then s precedes
t lexicographically if for some i: s_i\ <\ t_i and s_j = t_j for all j\ <\ i.
Sample input
1
9 3
alt
spy
sea
opt
pea
ape
spa
apt
ale
Sample output
ale alt apt opt
Source: BAPC preliminary contest, 2007