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

Given `"abcabcbb"`

, the answer is `"abc"`

, which the length is 3.

Given `"bbbbb"`

, the answer is `"b"`

, which the length is 3.

Given `"pwwkew"`

, 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.