---
title: '3. Longest Substring Without Repeating Characters'
description: Given a string s, find the length of the longest substring without duplicate characters
sidebar:
  label: 'Longest Substring Without Repeating Characters'
  badge: 'Medium'
---

<Badge variant="accent">Sliding Window</Badge>

### Example 1:
- Input: `s = "abcabcbb"`
- Output: `3`
- Explanation: The answer is "abc", with the length of `3`. Note that "bca" and "cab" are also correct answers.

### Example 2:
- Input: `s = "bbbbb"`
- Output: `1`
- Explanation: The answer is "b", with the length of `1`.

### Example 3:
- Input: `s = "pwwkew"`
- Output: `3`
- Explanation: The answer is "wke", with the length of `3`. Notice that the answer must be a substring, "pwke" is a subsequence and not a substring.

### Constraints:

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

## Solution

```py
class Solution:
    def lengthOfLongestSubstring(self, s: str) -> int:
        left = 0
        ans = 0
        seen = set()
        for right, c in enumerate(s):
            while c in seen:
                seen.remove(s[left])
                left += 1
            seen.add(c)
            ans = max(ans, right - left + 1)

        return ans
```