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Node Exists
On LockedIn, it's common for one user to navigate directly to another user's profile. We even creepily give the stalked user a notification that someone is looking at their profile.
To make this feature work, we need to be able to quickly check if a user exists in our tree.
Assignment
Complete the exists method.
It should take a value as input and return True if the value exists in the tree, and False if it doesn't. It's a recursive method, as you probably guessed.