3. Longest Substring Without Repeating Characters（HashSet + 双指针）

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

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

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.

class Solution {
    public int lengthOfLongestSubstring(String s) {
        int res = 0;
        if (s == null || s.length() == 0) return res; 
        //注意区别字符串的length()方法与数组的length属性
        HashSet<Character> set = new HashSet<>();
        for (int i = 0, j = 0; i < s.length();) {
            if (set.contains(s.charAt(i))) {
                set.remove(s.charAt(j++));
            } else {
                set.add(s.charAt(i++));
                res = Math.max(res, set.size());
            }
        }
        return res;
    }
}