题目：Given a string, find the length of the longest substring without repeating characters.
Examples:
Given "abcabcbb", the answer is "abc", which the length is 3.
Given "bbbbb", the answer is "b", with the length of 1.
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.
思路：用2个变量（left, right）分别保存子串的起点和终点。right自增，直到遇到重复字符为止；从重复字符出现的位置之后1位重新开始扫描。
class Solution { public int lengthOfLongestSubstring(String s) { HashSet<Character> hashSet = new HashSet<>(); int left = 0,right = 0; int maxLength = 0; while(right < s.length()){ if(!hashSet.contains(s.charAt(right))){ hashSet.add(s.charAt(right)); }else{ maxLength = Math.max(maxLength,right-left); while(s.charAt(left)!=s.charAt(right)){ hashSet.remove(s.charAt(left)); left++; } left++; //跳过重复的字符 } right++; } maxLength = Math.max(maxLength,right-left); return maxLength; } }