What exactly is leetcode? as opposed to just asking someone to prove they know the stuff listed on their resume. And suppose there are a lot of cheaters out th

What exactly is leetcode? as opposed to just asking someone to prove they know the stuff listed on their resume. And suppose there are a lot of cheaters out there who don't know the things on their resume, even including earning a degree without ever having to write a working program on their own. How would you screen them out?