題目：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"

思路：用2個變量（left, right）分別保存子串的起點和終點。right自增，直到遇到重復字符為止；從重復字符出現的位置之後1位重新開始掃描。

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++;    //跳過重復的字符
            }
            right++;
        }
        maxLength = Math.max(maxLength,right-left);
        return maxLength;
    }

}

[LeetCode] 3.Longest Substring Without Repeating Characters