• Java

Set<T> A = copyOf(a), B = copyOf(b);
List<T> c = new ArrayList<>();
for (T t : A) if (B.contains(t)) c.add(t);

• Java

a.retainAll(b);
List<T> c = new ArrayList<>(copyOf(a));

• Java

a.retainAll(b);
List<T> c = List.copyOf(Set.copyOf(a));

• Java

Set<T> A = copyOf(a), B = copyOf(b);
List<T> c = A.stream().filter(B::contains).toList();

• Clojure

(def c (clojure.set/intersection (set a) (set b)))

• C#

c = a.Intersect(b).ToList();

• Erlang

C = lists:flatten([A -- B] ++ [B -- A]).

• Go

func intersection[S ~[]T, T comparable](a, b S) S {
	s, l := a, b
	if len(b) < len(a) {
		s, l = b, a
	}

	set := make(map[T]struct{}, len(s))
	for _, x := range s {
		set[x] = struct{}{}
	}

	c := make(S, 0, len(s))
	for _, x := range l {
		if _, found := set[x]; found {
			c = append(c, x)
			delete(set, x)
		}
	}
	return c
}

• Go

func intersection[S ~[]T, T comparable](a, b S) S {
	seta := make(map[T]bool, len(a))
	for _, x := range a {
		seta[x] = true
	}
	setb := make(map[T]bool, len(b))
	for _, y := range b {
		setb[y] = true
	}

	var c S
	for x := range seta {
		if setb[x] {
			c = append(c, x)
		}
	}
	return c
}

• Go

seta := make(map[T]bool, len(a))
for _, x := range a {
	seta[x] = true
}
setb := make(map[T]bool, len(a))
for _, y := range b {
	setb[y] = true
}

var c []T
for x := range seta {
	if setb[x] {
		c = append(c, x)
	}
}

• Haskell

a `intersect` b

• JS

const c = [...new Set(a)].filter(e => b.includes(e));

• PHP

$c = array_intersect($a, $b)

• Pascal

for elem in a do
  if (b.indexof(elem) >= 0) and (c.indexof(elem) = -1) then
    c.add(elem);

• Perl

my %u;
$u{$_} = 1 for @a, @b;
my @c = keys %u;

• Python

c = list(set(a).intersection(b))

• Python

c = list(set(a) & set(b))

• Ruby

c = a & b

• Rust

let set_a: HashSet<_> = a.into_iter().collect();
let set_b: HashSet<_> = b.into_iter().collect();
let c = set_a.intersection(&set_b);

• Rust

let unique_a = a.iter().collect::<HashSet<_>>();
let unique_b = b.iter().collect::<HashSet<_>>();

let c = unique_a.intersection(&unique_b).collect::<Vec<_>>();