3.Longest Substring Without Repeating Characters

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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

思路：

用前后两个指针表示字符串中子串的一个滑动窗口；
右指针一直往前走，并把遍历过的每一个字符以及index都放入HashMap中，左指针根据当前右指针对应的字符是否出现过而调整位置（取靠右的位置）；
每遍历一个字符后判断更新最长不重复子串的最大值。

注意:

1.字符串长度为0时；
2.若字符在前面出现过，则取出现过的index+1与当前的左指针作比较；
3.若左右指针都是 从0开始，则长度为right-left+1；若左从0开始，右从1，则为right-left；