Sets: have I seen this?

Ask a list "is this value in you?" and it looks through its elements one by one to answer. A set answers the same question in one step, however many elements it holds.

1seen = set()
2for v in values:
3    if v in seen:
4        return True
5    seen.add(v)
6return False

At each element ask first ("have I seen this before?"), then add. A set holds each value only once; adding the same value twice changes nothing.

The price is memory: you keep what you have seen somewhere. In return two nested loops become one. That trade (give a little memory, gain a lot of time) is the whole of this topic.

Tasks