YouTube currently displays a like and a dislike button, allowing you to express your opinions about particular content. It's set up in such a way that you cannot like and dislike a video at the same time.
There are two other interesting rules to be noted about the interface:
Create a function that takes an array of button inputs and returns the final state.
likeOrDislike(["Dislike"]) ➞ "Dislike"
likeOrDislike(["Like", "Like"]) ➞ "Nothing"
likeOrDislike(["Dislike", "Like"]) ➞ "Like"
likeOrDislike(["Like", "Dislike", "Dislike"]) ➞ "Nothing"
"Nothing"."Nothing".