PROBLEM STATEMENT

A prefix-free set is a set of words such that no element in the set is a prefix of another element 
of the set. For example {"hello"} , {"hello", "goodbye", "giant", "hi"} and the empty set are 
examples of prefix-free sets. On the other hand, {"hello","hell"} and {"great","gig","g"} are not 
prefix-free.

You will be given a vector <string> words containing a set of words, and you must return the 
number of elements in the largest prefix-free subset of words.


DEFINITION
Class:PrefixFreeSets
Method:maxElements
Parameters:vector <string>
Returns:int
Method signature:int maxElements(vector <string> words)


NOTES
-A prefix of a string is the result of erasing zero or more characters from the right end of that 
string.


CONSTRAINTS
-words will contain between 1 and 50 elements, inclusive.
-Each element of words will contain between 1 and 50 characters, inclusive.
-Each character of each element of words will be a lowercase letter ('a'-'z').


EXAMPLES

0)
{"hello","hi","h","run","rerun","running"}

Returns: 4

{"hello","hi","run","rerun"} is a possible subset with 4 elements. Each subset having at least 5 
elements will always contain one of these forbidden pairs: "run" and "running", or "h" and "hi".

1)
{"a","b","cba","cbc","cbb","ccc"}

Returns: 6

This set is already prefix-free, so the best subset is itself, with 6 elements.

2)
{"a","ab","abc","abcd","abcde","abcdef"}

Returns: 1

Any pair of words is forbidden, so the best subset has only 1 element.

3)
{"topcoder","topcoder","topcoding"}

Returns: 2

"topcoder" is a prefix of "topcoder", so we can only have one of them in the subset.

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