Order is an important concept in mathematics and in computer science. For
example, Zorn's Lemma states: ``a partially ordered set in which every chain has
an upper bound contains a maximal element.'' Order is also important in
reasoning about the fix-point semantics of programs. This problem involves neither Zorn's Lemma nor fix-point semantics, but does
Given a list of variable constraints of the form x < y, you are to write a program that prints all orderings of the variables that are consistent with the constraints.
For example, given the constraints x < y and x < z there are two orderings of the variables x, y, and z that are consistent with these constraints: x y z and x z y.
x y z
x z y
The input consists of a sequence of constraint specifications. A
specification consists of two lines: a list of variables on one line followed by
a list of contraints on the next line. A constraint is given by a pair of
variables, where x y indicates that x < y.
All variables are single character, lower-case letters. There will be at
least two variables, and no more than 20 variables in a specification. There
will be at least one constraint, and no more than 50 constraints in a
specification. There will be at least one, and no more than 300 orderings
consistent with the contraints in a specification.
Input is terminated by end-of-file.
第二行 两个两个一组，(x y) 表示（x<y）.至少一组条件且不起过300组条件。
For each constraint specification, all orderings consistent with the
constraints should be printed. Orderings are printed in lexicographical
(alphabetical) order, one per line.
Output for different constraint specifications is separated by a blank line.