Question 1

Set membership test

Accepted Answer

Sets only store unique elements, so adding 2 a second time has no effect - the set already contains it. Adding 4 does insert a new element, bringing the count from 3 to 4. This is the core property of sets: duplicates are silently ignored.

Question 2

List append() complexity

Accepted Answer

Python lists over-allocate storage, so most appends just write to an already-reserved slot in O(1). Occasionally the buffer fills and Python copies the list to a larger allocation - that copy is O(n), but it happens so rarely that the cost spreads out to O(1) per append on average. The key word is "amortized" - the worst case exists, but it is not representative of typical behavior.

Question 3

Dictionary missing key behavior

Accepted Answer

d.get(key, default) returns the default when the key is absent instead of raising a KeyError. Since "c" is not in d, the call returns 99. Using .get() with a default is the idiomatic way to handle optional keys - it keeps the code clean and avoids a try/except block for a very common case.