What does s[i] return in Ruby?

A new one-character String, such as "a", not an integer code (that changed back in Ruby 1.9). Use s.ord or s.getbyte(i) for the number. On a non-ASCII string s[i] has to scan from the start to find the i-th character, which is why the solutions call s.chars once or walk with each_char.

Does it work with UTF-8 strings like "café☕é"?

Yes. Ruby strings know their encoding, and chars and each_char yield characters (code points), not bytes, so all three versions return 5 ("café☕"). An accent written as a separate combining mark is its own code point; each_grapheme_cluster groups those if you need what a reader sees as one symbol.

Do I need require "set"?

Not on Ruby 3.2 or later, where Set is loaded automatically. On older versions add require 'set' at the top. A Hash with true values (seen[c] = true, seen.key?(c)) works on every version if you prefer no Set at all.

Does [best, x].max allocate an array every time?

No. CRuby compiles a literal [a, b].max to a special instruction that compares the values without building the array, so it is as cheap as best = x if x > best. Both forms are idiomatic.

How do I return the substring instead of its length?

Remember where the best window starts: start, best = left, right - left + 1 if right - left + 1 > best, then return s[start, best], which takes best characters from start. That gives "etcar" for "streetcar" and " world" for "hello world"; the strict > keeps the first of several equally long windows, so "noon" gives "no".

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

length_of_longest_substring.rb

Ruby 4.0

Not run yet

+ 8 hidden

Input

s = "streetcar"

Expected

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