- Checking existence after insertion: In Two Sum, check if complement exists before adding current element to avoid matching element with itself.
- Using wrong key type: Mutable objects as keys cause bugs.
Use immutable types like strings, tuples, or frozen sets.
- Not handling hash collisions mentally: While hash maps handle collisions internally, understanding worst-case helps in interviews.
- Forgetting to initialize counters: Use defaultdict or check existence before incrementing to avoid KeyError.