GET is idempotent because every time it returns a fixed or same result.
PUT is also idempotent because it can update multiple times and return the same result every time.
POST is not idempotent because every time it creates new item and return different item.
DELETE is not idempotent because after deleting an item it will return null for that same item.
Source Link : http://www.restapitutorial.com/lessons/idempotency.html
Source video : https://www.youtube.com/watch?time_continue=246&v=6dVNdFwqeKs