18. Longest Substring Without Repeating Characters

mediumAsked at Glassdoor

Extracting the longest unique-word run from a review snippet is analogous to what Glassdoor's NLP team does daily — this sliding-window problem tests whether you can maintain a dynamic window without backtracking on every character.

By Alex Chen, Founder, InterviewChamp.AI · Last verified 2026-05-26

Problem

Given a string s, find the length of the longest substring that contains no repeating characters.

Constraints

0 <= s.length <= 5 * 10^4
s consists of English letters, digits, symbols and spaces

Examples

Example 1

Input

s = "abcabcbb"

Output

Explanation: "abc" is the longest substring without repeating characters, with length 3.

Example 2

Input

s = "bbbbb"

Output

Explanation: "b" is the only non-repeating substring.

Approaches

1. Brute force — check all substrings

Generate every substring and check for duplicates using a Set. O(n^3) — too slow beyond a few hundred characters.

Time: O(n^3)
Space: O(min(n, m))

function lengthOfLongestSubstring(s) {
  let max = 0;
  for (let i = 0; i < s.length; i++) {
    for (let j = i + 1; j <= s.length; j++) {
      const set = new Set(s.slice(i, j));
      if (set.size === j - i) max = Math.max(max, j - i);
    }
  }
  return max;
}

Tradeoff:

2. Sliding window with hash map

Maintain a left pointer and a map of character → last-seen index. When a repeat is found, jump left past the previous occurrence. Single pass O(n).

Time: O(n)
Space: O(min(n, m))

function lengthOfLongestSubstring(s) {
  const last = new Map();
  let left = 0;
  let max = 0;
  for (let right = 0; right < s.length; right++) {
    const c = s[right];
    if (last.has(c) && last.get(c) >= left) {
      left = last.get(c) + 1;
    }
    last.set(c, right);
    max = Math.max(max, right - left + 1);
  }
  return max;
}

Tradeoff:

Glassdoor-specific tips

Glassdoor interviewers use this problem to probe whether you can reason about two-pointer invariants without drawing them out. Emphasize that 'left' only ever moves forward — it never resets — which is the key insight that keeps this linear. They also like seeing you handle the edge case where the stored index of a repeated character is to the left of the current window, which is why the `>= left` guard matters.

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Output

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