Longest substring without repeating characters

Medium25 minFree, no account

The canonical sliding window, and the one-line optimisation that makes it genuinely linear.

The question

Given a string, return the length of the longest substring with no repeated characters.

Then return the substring itself.

longest("abcabcbb")   // 3   "abc"
longest("bbbbb")      // 1   "b"
longest("pwwkew")     // 3   "wke", substring, not subsequence
longest("")           // 0

25:00Commit to an answer before you open the solution. Reading it first teaches you to recognise good answers, which is not the skill being tested.

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The worked solution

written by a person · not a grade

Score yourself

0 of 5 marked

A correct sliding window in one pass30
Handled the previous-occurrence-outside-window case30
Returned the substring as well as the length15
Justified O(n) rather than asserting it15
Handled empty and single-character input10

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