There are several ways to pick out letters from the sample text - every second letter, every third letter, every fourth letter, etc. You want to build each of those only once, rather than building each one over again for every word in the list. You want to only consider arrangements that have a real chance - if the text is only 400 characters, and all the words in the list are at least 4 characters, then considering “every 200th letter” is a waste of time.
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