Implement a Snapshot Set Iterator

Design and implement a set data structure that supports snapshot iteration.

The set stores unique values. It must support the following operations:

• add(value): Add value to the set if it is not already present.
• remove(value): Remove value from the set if it is present. If it is absent, do nothing.
• contains(value) -> bool: Return whether value is currently in the set.
• iterator(): Return an iterator over a snapshot of the set at the moment the iterator is created.

Snapshot iterator requirements:

• The iterator should return exactly the values that were present in the set when the iterator was created.
• Later calls to add or remove must not affect already-created iterators.
• The return order of values is not important.
• Each value in the snapshot should be returned exactly once.

Example:

s = SnapshotSet()
s.add(1)
s.add(2)
it = s.iterator()
s.remove(1)
s.add(3)

# The iterator should still return 1 and 2 in any order.
# It should not return 3.

Implement the data structure and iterator. Discuss the time and space complexity of your approach.