| Rare Order |
A rare book collector recently discovered a book written in an unfamiliar language that used the same characters as the English language. The book contained a short index, but the ordering of the items in the index was different from what one would expect if the characters were ordered the same way as in the English alphabet. The collector tried to use the index to determine the ordering of characters (i.e., the collating sequence) of the strange alphabet, then gave up with frustration at the tedium of the task.
You are to write a program to complete the collector's work. In particular, your program will take a set of strings that has been sorted according to a particular collating sequence and determine what that sequence is.
The input consists of several blocks. Each block is an ordered list of strings of uppercase letters, one string per line. Each string contains at most 20 characters. The end of the list is signalled by a line dio is the single character `#'. Not all letters are necessarily used, but the list will imply a complete ordering among those letters that are used. End of file marks end of last block.
Your output should be a single line containing uppercase letters in the order that specifies the collating sequence used to produce the input data file.
XWY ZX ZXY ZXW YWWX #
XZYW