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

# 3.Longest-Substring-Without-Repeating-Characters

## 3. Longest Substring Without Repeating Characters

## 题目地址

<https://leetcode.com/problems/longest-substring-without-repeating-characters/>

## 题目描述

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

Example 1:
Input: "abcabcbb"
Output: 3 
Explanation: The answer is "abc", with the length of 3. 

Example 2:
Input: "bbbbb"
Output: 1
Explanation: The answer is "b", with the length of 1.

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

## 代码

### Approach 1: Sliding Window

**Complexity Analysis**

* Time complexity : `O(2n)`
* Space complexity : `O(min(m, n))`

```java
class Solution {
  public int lengthOfLongestSubstring(String s) {
    int n = s.length();
    Set<Character> set = new HashSet<>();
    int ans = 0, i = 0, j = 0;
    while (i < n && j < n) {
      if (!set.contains(s.charAt(j))) {
        set.add(s.charAt(j++));
        ans = Math.max(ans, j - i);
      } else {
        set.remove(s.charAt(i++));
      }
    }

    return ans;
  }
}
```

### Approach 2: Sliding Window Optimized

map: Character => Index

conatians character => update `i`

```java
class Solution {
  public int lengthOfLongestSubstring(String s) {
    int n = s.length(), ans = 0;
    Map<Character, Integer> map = new HashMap<>();

    for (int j = 0, i = 0; j < n; j++) {
      if (map.containsKey(s.charAt(j))) {
        i = Math.max(map.get(s.charAt(j)) + 1, i);
      }
      ans = Math.max(ans, j - i + 1);
      map.put(s.charAt(j), j); 
    }

    return ans;
  }
}
```

用 `int[128]` 代替 map: Character => index

```java
class Solution {
  public int lengthOfLongestSubstring(String s) {
    int n = s.length(), ans = 0;
    int[] index = new int[128];
    for (int j = 0, i = 0; j < n; j++) { abbcde
      i = Math.max(index[s.charAt(j)], i);
      ans = Math.max(ans, j - i + 1);
      index[s.charAt(j)] = j + 1; // 为什么 j+1 => 当遇到i元素去重后, 取下一位
    }

    return ans;
  }
}
```

### Approach 3: Brute Force

**Intuition**

Check all the substring one by one to see if it has no duplicate character.

**Complexity**

* Time complexity: O(n^3)
* Space complexity: O(min(n,m)), The size of the Set is upper bounded by the size of the string *n* and the size of the charset/alphabet *m*.

```java
class Solution {
    public int lengthOfLongestSubstring(String s) {
    int n = s.length();
    int ans = 0;
    for (int i = 0; i < n; i++) {
      for (int j = i + 1; i <= n; j++) {
        if (allUnique(s, i, j)) {
          ans = Math.max(ans, j - i);
        }
      }
    }

    return ans;
  }

  public boolean allUnique(String s, int start, int end) {
    Set<Character> set = new HashSet<>();
    for (int i = start; i < end; i++) {
      Character ch = s.charAt(i);
      if (set.contains(ch)) return false;
      set.add(ch);
    }
    return true;
  }

}
```


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://wentao-shao.gitbook.io/leetcode/string/3.longest-substring-without-repeating-characters.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.