Common Mistakes That Kill Interview Scores

Concepts: pyMutableDefault, pyEdgeCases, pyFreqOfFreq, pyZeroCount

Frequency counting problems are deceptively simple. Candidates think they know them, write code quickly, and then get tripped up by one of five recurring bugs. Each one is a signal to the interviewer that the candidate either does not handle edge cases systematically or does not fully understand Python's data model. Know these cold. They take 10 seconds to prevent and 10 minutes to debug under pressure. Mistake 1: Mutable Default Argument with dict Mistake 2: Forgetting Empty Input and Single-Element Input Mistake 3: Confusing Frequency of Frequencies This is the conceptually tricky one. 'Frequency of frequencies' means counting how many elements appear exactly k times. Counter(Counter(data).values()) gives you that. Candidates often confuse the two levels and write code that answers the w