The comment you're replying to is replying to a comment about using a hash map; I read it as a suggestion to replace that with a hash set, which would be O(n) i

The comment you're replying to is replying to a comment about using a hash map; I read it as a suggestion to replace that with a hash set, which would be O(n) in time & space for the article's problem. (You don't need the "value" half of the hash map for this problem.)