How Hash Sets Work — API Cheat Sheet

This is a reference page, not a graded exercise. A set is a hash map that only stores keys, no values — reach for it whenever a problem is really asking "is this a duplicate" or "have I visited this before," which is a very common pattern.

Java — HashSet<T>

Want to...	Method	Example
Create	new HashSet<>()	Set<Integer> s = new HashSet<>();
Add	.add(v)	returns false if it was already present
Check membership	.contains(v)	s.contains(5)
Remove	.remove(v)
Size	.size()
Iterate	for-each	for (int x : s) — no guaranteed order
Build from a collection	new HashSet<>(list)	instant de-duplication

C# — HashSet<T>

Want to...	Method	Example
Create	new HashSet<T>()	var s = new HashSet<int>();
Add	.Add(v)	returns false if it was already present
Check membership	.Contains(v)	s.Contains(5)
Remove	.Remove(v)
Size	.Count	property, no parens
Set operations	.UnionWith(other), .IntersectWith(other), .ExceptWith(other)	mutate s in place

Python — set

Want to...	Method	Example
Create	set()	not {} — that's an empty dict
Create with values	set literal	s = {1, 2, 3}
Add	.add(v)
Check membership	in	5 in s
Remove	.remove(v) (raises if missing) / .discard(v) (silent)	prefer .discard if you're not sure it's there
Size	len(s)	builtin
Set operations	s1 | s2 (union), s1 & s2 (intersection), s1 - s2 (difference)	operators, not just methods
Build from a list	set(my_list)	instant de-duplication

JavaScript / TypeScript — Set<T>

Want to...	Method	Example
Create	new Set()	const s = new Set<number>();
Add	.add(v)
Check membership	.has(v)	s.has(5)
Remove	.delete(v)
Size	.size	property, no parens
Iterate	for (const x of s)	preserves insertion order
Build from an array	new Set(arr)	instant de-duplication
Back to an array	[...s] or Array.from(s)

The gotcha that costs the most time: Python's {} is an empty dict, not an empty set — you must write set() explicitly. And across every language, sets compare objects/structs by reference unless you're using primitives or the language's built-in value types (strings, numbers) — putting two "equal-looking" custom objects into a set won't dedupe them unless you've defined equality/hashing for that type.