Longest Substring Without Repeats

Find the longest stretch of s that has no repeated character, and return its length. A stretch here means a substring: characters that sit side by side in s.

• s may hold any single characters: letters, digits, spaces, punctuation and so on.
• Upper and lower case are different characters, so "a" and "A" don't clash.
• An empty string gives 0.
• s can be up to 100,000 characters long, and the answer can be long too.