Q2

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.