> For the complete documentation index, see [llms.txt](https://mayanktyagi3111.gitbook.io/interview-prep/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://mayanktyagi3111.gitbook.io/interview-prep/hashmap-and-hashset-and-sliding-window/longest-substring-without-repeating-characters.md).

# Longest Substring Without Repeating Characters

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.
```

```java
class Solution {
    public int lengthOfLongestSubstring(String s) {
        int best_so_far = 0;
        HashMap<Character, Integer> map = new HashMap<>();
        int start = 0;
        for (int i = 0; i < s.length(); i++) {
            if (map.containsKey(s.charAt(i))) {
                if (map.get(s.charAt(i)) >= start) {
                    start = map.get(s.charAt(i)) + 1; // Because the previous character ( which was same) will not be
                                                      // considered while counting the new
                                                      // length
                }
            }
            best_so_far = Math.max(best_so_far, i - start + 1);
            map.put(s.charAt(i), i);
        }
        return best_so_far;
    }
}
```