What are you "giving away" exactly? If you're faced with a day of random algorithm challenges, you'll naturally stumble on some and excel on others.
A good interviewer is looking for how you explain what you're doing, and how you come up with the solution.
A bad interviewer wants to stump and stress you... but in the case of a bad interview, who cares what is legit or not.
Well, I have a horrifying scenario in my mind. I mean, assume there were 5 problems overall -- two very hard, two medium hard, and one easy. Now if the interviewee just breezes through the two hard ones but isn't able to make headway in the other, it's bound to raise questions.
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