17. Longest Substring Without Repeating Characters

mediumAsked at Indeed

Find the length of the longest substring without repeating characters — the sliding window pattern powers Indeed's query tokenizer deduplication.

By Alex Chen, Founder, InterviewChamp.AI · Last verified 2026-05-20

Problem

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

Constraints

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

Examples

Example 1

Input

s = "abcabcbb"

Output

Example 2

Input

s = "bbbbb"

Output

Approaches

1. Brute force

Check every substring with a Set to detect duplicates.

Time: O(n^2)
Space: O(min(n,m))

function lengthOfLongestSubstring(s) {
  let max = 0;
  for (let i = 0; i < s.length; i++) {
    const seen = new Set();
    for (let j = i; j < s.length; j++) {
      if (seen.has(s[j])) break;
      seen.add(s[j]);
      max = Math.max(max, j - i + 1);
    }
  }
  return max;
}

Tradeoff:

2. Sliding window with last-seen map

Track the last index of each character; jump the left pointer directly past the duplicate to avoid inner-loop scans.

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

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

Tradeoff:

Indeed-specific tips

Indeed's search team frequently asks this; connect the window technique to streaming token deduplication in their job-title indexer to demonstrate domain awareness.

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Output

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