most-frequently-used-words-in-a-text

Most frequently used words in a text

Write a function that, given a string of text (possibly with punctuation and line-breaks), returns an array of the top-3 most occurring words, in descending order of the number of occurrences.

Assumptions:

• A word is a string of letters (A to Z) optionally containing one or more apostrophes (') in ASCII.
• Apostrophes can appear at the start, middle or end of a word ('abc, abc', 'abc', ab'c are all valid)
• Any other characters (e.g. #, \, / , . ...) are not part of a word and should be treated as whitespace.
• Matches should be case-insensitive, and the words in the result should be lowercased.
• Ties may be broken arbitrarily.
• If a text contains fewer than three unique words, then either the top-2 or top-1 words should be returned, or an empty array if a text contains no words.

Examples:

top_3_words("In a village of La Mancha, the name of which I have no desire to call to
mind, there lived not long since one of those gentlemen that keep a lance
in the lance-rack, an old buckler, a lean hack, and a greyhound for
coursing. An olla of rather more beef than mutton, a salad on most
nights, scraps on Saturdays, lentils on Fridays, and a pigeon or so extra
on Sundays, made away with three-quarters of his income.")
# => ["a", "of", "on"]

top_3_words("e e e e DDD ddd DdD: ddd ddd aa aA Aa, bb cc cC e e e")
# => ["e", "ddd", "aa"]

top_3_words("  //wont won't won't")
# => ["won't", "wont"]

For java users, the calls will actually be in the form: TopWords.top3(String s), expecting you to return a List<String>.

Bonus points (not really, but just for fun):

1. Avoid creating an array whose memory footprint is roughly as big as the input text.
2. Avoid sorting the entire array of unique words.