Problem: Longest Substring without repeating.
Problem  description:  Given a string, find the length of the longest substring without repeating characters. For example, the longest substring without repeating letters for "abcabcbb" is "abc", which the length is 3. For "bbbbb" the longest substring is "b", with the length of 1.

Came out with the good solution and complete the code within 20 mins, but have bugs, not simple enough.