Why a while loop instead of break in the brute force?

A for body is a closure, and leaving it with return is a non-local return that Scala 3 warns about on every compile. Folding the stop condition into while (j < s.length && seen.add(s(j))) needs no early exit at all. import scala.util.boundary, boundary.break with boundary { ... break() } around the inner loop is the other warning-free option.

What type is s(i) in Scala?

s(i) goes through StringOps.apply and is the same as s.charAt(i): a Char, one UTF-16 code unit, read in O(1). So an emoji like 😀, which is a surrogate pair, counts as two characters; for code points, run the window over s.codePoints.toArray with a mutable.HashMap[Int, Int].

Why does the match need case _ =>?

Without it, None and Some(prev) with prev makes those cases a deliberate no-op. lastSeen.get(c).filter(_ >= left).foreach(p => left = p + 1) is a match-free alternative.

Is a mutable collection idiomatic here?

Inside one method, yes: the set or map never escapes, and updating in place avoids building a new immutable collection on every character. A purely functional version folds over s.indices with an immutable Map and a (left, best) tuple; it is correct but allocates far more.

Can a fixed Array replace the HashMap?

Yes, and it is faster. val last = new Array[Int](128) holding right + 1 (0 = not seen) turns the body into left = math.max(left, last(c)); last(c) = right + 1, where c widens from Char to Int automatically. It throws ArrayIndexOutOfBoundsException on a character above 127 such as é, so new Array[Int](65536) is the safe size for any Char.

Longest Substring Without Repeating Characters in Scala: Brute Force and Sliding Window Solutions

Q: What type is s(i) in Scala?

s(i) goes through StringOps.apply and is the same as s.charAt(i): a Char, one UTF-16 code unit, read in O(1). So an emoji like 😀, which is a surrogate pair, counts as two characters; for code points, run the window over s.codePoints.toArray with a mutable.HashMap[Int, Int].

length_of_longest_substring.scala

Scala 3.6

Not run yet

+ 8 hidden

Input

s = "streetcar"

Expected

The double e breaks the word up. After it, "etcar" has five different characters.