Longest Substring Without Repeating…

1. Define the problem

Longest Substring Without Repeating Characters

Given a string s, return the length of the longest substring without repeating characters . Grow a window on the right and shrink from the left whenever a duplicate enters, so the window never holds the same character twice.

Constraints

0 ≤ s.length ≤ 5 × 10⁴
s consists of English letters, digits, symbols and spaces

Example

Inputs = "abcabcbb"

Output3

Explanation The answer is "abc", with length 3.

2. Visualize the solution

Expand until a duplicate, then shrink left

Statusgrow

Window [a]; best = 1.

What happens in this step

window = [0, 0]  ("a")

before: seen = {}, best = 0
  add s[0] = 'a' → seen = {a}

seen now holds just 'a'; the window length 1 becomes the new best.

Step 1 of 5

Steps to visualize

Grow right and add each new character into a seen set.
When the new char is already in the set, shrink left until the duplicate is gone.
After each add, the window holds only unique characters.
Track the longest valid window length as best.
Continue until right reaches the end of the string.

3. Walk through the code

Walk through the code

Same walkthrough, now with the code. Press Next to move one step and watch which lines run.

Expand until a duplicate, then shrink left

Statusgrow

Window [a]; best = 1.

What happens in this step

window = [0, 0]  ("a")

before: seen = {}, best = 0
  add s[0] = 'a' → seen = {a}

seen now holds just 'a'; the window length 1 becomes the new best.

Step 1 of 5

4. Solution

Solution

solution.tsTypeScript

function lengthOfLongestSubstring(s) {
  const seen = new Set();
  let left = 0;
  let best = 0;

  for (let right = 0; right < s.length; right++) {
    const c = s[right];
    while (seen.has(c)) {
      seen.delete(s[left]);
      left++;
    }
    seen.add(c);
    best = Math.max(best, right - left + 1);
  }

  return best;
}

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

5. Test cases

Test cases

Input	Expected	Covers
`s = "abcabcbb"`	`3`	Docstring example — longest unique window is "abc"
`s = ""`	`0`	Empty string has length 0
`s = "a"`	`1`	Single character
`s = "abcdef"`	`6`	Entire string is unique
`s = "bbbbb"`	`1`	All identical characters
`s = "pwwkew"`	`3`	Classic pwwkew case — "wke"
`s = "dvdf"`	`3`	Must shrink past first d to take "vdf"
`s = "a b c a"`	`3`	Spaces and repeats far apart

Topic	Description
Sliding Window	Solve fixed-window problems: maximum sum of k elements, moving averages, and O(n) updates as the window slides.
Maximum Average Subarray I	Find the contiguous subarray of a fixed length with the largest average.
Permutation in String	Check whether one string contains a rearrangement of another string as a substring.
Minimum Size Subarray Sum	Find the shortest contiguous subarray whose sum is at least a target value.
Longest Repeating Character Replacement	Find the longest substring you can turn into one repeated letter with a limited number of swaps.
Fruit Into Baskets	Find the longest run of fruit you can collect while carrying only two types at once.