# GeetCode Hub

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

Example 1:

```Input: s = "abcabcbb"
Output: 3
Explanation: The answer is "abc", with the length of 3.
```

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.
```

Example 4:

```Input: s = ""
Output: 0
```

Constraints:

• `0 <= s.length <= 5 * 104`
• `s` consists of English letters, digits, symbols and spaces.

public int LengthOfLongestSubstring(string s) { int max = 0; int start = 0; int end; var hashSet = new HashSet<char>(); for (int i = 0; i < s.Length; i++) { if (hashSet.Contains(s[i])) { while (s[start] != s[i]) { hashSet.Remove(s[start]); start++; } start++; } else { hashSet.Add(s[i]); if (max < hashSet.Count) { max = hashSet.Count; } } } return max; }

The basic idea of solving this problem is to keep track of all the characters we have already visited and maintain a window size in which each character appeared only once. So throughout the traversal whenever the window size is maximum, that will give us the result.

Time Complexity: O(n)  // For traversing all the items in the array.

Space Complexity: O(n) // for keeping track of already visited items in the window, in the worst case all items will be present in the HashSet.

Medium