4kyu Secret Phrase Problem

[simon-tiger]

⚠NOTE⚠: This kata is reworded, because I used this wording in my not-published-yet video on the problem.

You need to guess a secret phrase. You know the following about it:

1. A sequence of triplets [a, b, c], such that, for every triplet, a comes before b comes before c in the phrase.
2. The letters that appear in the triplets are the only letters in the phrase.
3. Each letter in the phrase appears once and only once in the phrase.

Input: The sequence of triplets
Output: The secret phrase
Example:

Input                 Output
[["t", "u", "p"],     "whatisup"
 ["w", "h", "i"],
 ["t", "s", "u"],
 ["a", "t", "s"],
 ["h", "a", "p"],
 ["t", "i", "s"],
 ["w", "h", "s"]]

Link: https://www.codewars.com/kata/53f40dff5f9d31b813000774