Here is the interview question prompt, presented for reference.

A very common interview challenge is to determine how often words appear in a given string or set of strings. Here's a version of this: return a list of duplicate words in a sentence.

For example, given 'The dog is the best', returns ["the"].

Likewise, given 'Happy thanksgiving, I am so full', you would return an empty array. This is because there are no duplicates in the string.

Constraints

• The total length of the string <= 100000
• The string will consist of ASCII characters (may be some or all)
• Expected time complexity : O(n) where n are the number of words in the string
• Expected space complexity : O(n)

You can see the full challenge with visuals at this link.