LeetCode 3 Longest Substring Without Repeating Characters 最長的無重複字符的子字串

本篇為LeetCode上演算法的Medium問題，Longest Substring Without Repeating Characters 最長的無重複字符的子字串。

題目如下：

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

Example 1:

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

Example 2:

  Input: "bbbbb"
  Output: 1
  Explanation: The answer is "b", with the length of 1.

Example 3:

  Input: "pwwkew"
  Output: 3
  Explanation: 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.