Word Frequency Algorithm

Project Description

[updated to add more clarity on the problem statement]

We have a large set of sentences. 

I 'm looking for the algorithm that will produce the *minimum* set of words (in descending order of appearence) such that each sentence contains *at least one* of these words.

For example:

For the sentence set {"black white red", "black white"} the minimum set of words is 

"black"

For the sentence set {"one two five", "three four six", "seven nine one"} the minimum set of words is

"one", "three"

(there are other correct answers, too, but none with less than 2 words)