> For the complete documentation index, see [llms.txt](https://chunhthanhde.gitbook.io/leetcode-top-interview/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://chunhthanhde.gitbook.io/leetcode-top-interview/topic-3-sliding-window/031-longest-substring-without-repeating-characters.md).

# 3. Longest Substring Without Repeating Characters 🌐

**Difficulty**: `Medium` - **Tags**: `Hash Table`, `String`, `Sliding Window`

[LeetCode Problem Link](https://leetcode.com/problems/longest-substring-without-repeating-characters/)

## Problem Statement 📜

Given a string `s`, find the length of the longest substring without repeating characters.

## Examples 🌟

🔹 **Example 1:**

**Input:**

```python
s = "abcabcbb"
```

**Output:**

```
3
```

**Explanation:** The answer is "abc", with the length of 3.

🔹 **Example 2:**

**Input:**

```python
s = "bbbbb"
```

**Output:**

```
1
```

**Explanation:** The answer is "b", with the length of 1.

🔹 **Example 3:**

**Input:**

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

## Solution 💡

To solve this problem, we can use the **Sliding Window** approach:

1. **Use a Set**: Utilize a `HashSet` to track characters in the current window.
2. **Expand and Shrink the Window**:
   * Expand the `right` pointer to include characters in the window.
   * If a duplicate character is found, shrink the window from the left until the window contains no duplicate characters.
3. **Update the Result**:
   * Keep track of the maximum length of valid substrings.

### Java Solution

```java
import java.util.HashSet;

public class Solution {
    public int lengthOfLongestSubstring(String s) {
        HashSet<Character> charSet = new HashSet<>();
        int left = 0, maxLength = 0;

        for (int right = 0; right < s.length(); right++) {
            while (charSet.contains(s.charAt(right))) {
                charSet.remove(s.charAt(left));
                left++;
            }
            charSet.add(s.charAt(right));
            maxLength = Math.max(maxLength, right - left + 1);
        }

        return maxLength;
    }
}
```

## Time Complexity ⏳

* **O(n)**: Each character is processed at most twice (once added, once removed).

## Space Complexity 💾

* **O(min(n, m))**: Space used by the `HashSet`, bounded by the size of the input `n` and the size of the character set `m`.

You can find the full solution [here](https://github.com/ChunhThanhDe/Leetcode-Top-Interview/blob/main/Topic%203%20Sliding%20Window/031%20Longest%20Substring%20Without%20Repeating%20Characters/Solution.java).