Have we seen this before?
Picture
Keep a small checklist of places you’ve visited. Before adding the park, ask whether it’s already there. A set keeps one copy of each distinct item.
Predict
Add the park twice to a set. How many copies of the park are there?
Let's look
One. Adding an existing item doesn’t create another copy.
A little Python, if you want it
seen = set()
seen.add("park")
seen.add("park")
assert "park" in seen
assert len(seen) == 1in asks whether an item is present. A set is useful for membership, not for remembering the order of a route. Python set membership is expected constant time for ordinary well-behaved keys.
Change one rule
We also need to remember how many times we visited the park. Is membership alone enough?
Let's try it
No. We need to attach a count to the place. A dictionary can do that.
Connect
Next, attach a value to a name. Later, visited sets help searches avoid unwanted repeats.