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14. Longest Substring Without Repeating Characters

mediumAsked at Ramp

Find the length of the longest substring with all unique characters.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given a string s, find the length of the longest substring without repeating characters. A substring is a contiguous sequence of characters.

Constraints

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

Examples

Example 1

Input
s = "abcabcbb"
Output
3

Example 2

Input
s = "pwwkew"
Output
3

Approaches

1. Brute force

Check every substring for uniqueness using a set.

Time
O(n^3)
Space
O(n)
function lengthOfLongestSubstring(s) {
  let best = 0;
  for (let i = 0; i < s.length; i++) {
    const seen = new Set();
    for (let j = i; j < s.length && !seen.has(s[j]); j++) {
      seen.add(s[j]);
      best = Math.max(best, j - i + 1);
    }
  }
  return best;
}

Tradeoff:

2. Sliding window with last-seen map

Track the last index each character was seen; when a duplicate falls inside the window, jump the left pointer past it.

Time
O(n)
Space
O(k)
function lengthOfLongestSubstring(s) {
  const last = new Map();
  let left = 0, best = 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);
    best = Math.max(best, right - left + 1);
  }
  return best;
}

Tradeoff:

Ramp-specific tips

Ramp uses sliding-window patterns inside their fraud-detection layer to spot duplicate-card runs without scanning the full transaction log; signaling you understand last-seen pruning earns bonus.

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Output

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