Problem 2: First! [Mark Gordon, 2012]

Bessie has been playing with strings again. She found that by
changing the order of the alphabet she could make some strings come
before all the others lexicographically (dictionary ordering).

For instance Bessie found that for the strings "omm", "moo", "mom", and
"ommnom" she could make "mom" appear first using the standard alphabet and
that she could make "omm" appear first using the alphabet
"abcdefghijklonmpqrstuvwxyz". However, Bessie couldn't figure out any way
to make "moo" or "ommnom" appear first.

Help Bessie by computing which strings in the input could be
lexicographically first by rearranging the order of the alphabet. To
compute if string X is lexicographically before string Y find the index of
the first character in which they differ, j. If no such index exists then
X is lexicographically before Y if X is shorter than Y. Otherwise X is
lexicographically before Y if X[j] occurs earlier in the alphabet than Y[j].

PROBLEM NAME: first

INPUT FORMAT:

• Line 1: A single line containing N (1 <= N <= 30,000), the number of
  strings Bessie is playing with.

• Lines 2..1+N: Each line contains a non-empty string. The total
  number of characters in all strings will be no more than
  300,000. All characters in input will be lowercase characters
  'a' through 'z'. Input will contain no duplicate strings.

SAMPLE INPUT:

4
omm
moo
mom
ommnom

INPUT DETAILS:

The example from the problem statement.

OUTPUT FORMAT:

• Line 1: A single line containing K, the number of strings that could
  be lexicographically first.

• Lines 2..1+K: The (1+i)th line should contain the ith string that
  could be lexicographically first. Strings should be output in
  the same order they were given in the input.

SAMPLE OUTPUT:

2
omm
mom

OUTPUT DETAILS:

Only "omm" and "mom" can be ordered first.

见题面

见题面