3\. Longest Substring Without Repeating Characters

Medium

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:**

*   <code>0 <= s.length <= 5 * 10<sup>4</sup></code>
*   `s` consists of English letters, digits, symbols and spaces.