Given a string with words separated by spaces, return an array with the words that appear exactly once, in the same order they first appear in the string.

The comparison must be case-insensitive (i.e., "Hello" and "hello" are considered the same word), but the words in the result must appear exactly as they were found the first time.

Examples

uniqueWords("hello world hello");          // ["world"]
uniqueWords("one two three two one");      // ["three"]
uniqueWords("a b c d");                    // ["a", "b", "c", "d"]
uniqueWords("Hello hello HELLO");          // []
uniqueWords("");                           // []

Notes

• If there are no unique words, return an empty array [].
• The string can be empty.
• There will be no punctuation, only words and spaces.