#3 Longest Substring Without Repeating Characters
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#3 Longest Substring Without Repeating Characters
问题描述
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.
来源：力扣（LeetCode）
链接：https://leetcode-cn.com/problems/longest-substring-without-repeating-characters
著作权归领扣网络所有。商业转载请联系官方授权，非商业转载请注明出处。
解题思路
利用滑动窗口和哈希map。
首先对字字符串进行遍历，每读取一个元素，都要找到上一个与当前元素重复的index，记为flag。利用字典储存某个元素及其最后出现的位置。
时间复杂度：(O(n))，依次遍历
空间复杂度：(O(1))，常数级存储空间用于储存字符串
代码解释
line 9:flag从-1开始记录，因为第一个元素开始 的长度为(0-(-1)=1)
line 19:只有在存入新字符的时候才会更新最长不重复子串的常数
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