鏈接：https://leetcode.com/problems/longest-substring-without-repeating-characters/
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.
Example 3:
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.

（1）滑動(dòng)窗口思想
滑動(dòng)窗口為[i，j-1]，如果滑動(dòng)窗口中存在字符（位置k）與j位置的字符相同，則從set集合中刪除[i，k]之間的所有字符；如果滑動(dòng)窗口中不存在字符與j位置的字符相同，則將j位置的字符加入set集合。

    public int lengthOfLongestSubstring(String s) {
        int n = s.length();
        Set<Character> set = new HashSet<>();
        int ans = 0, i = 0, j = 0;
        while (i < n && j < n) {
            if (!set.contains(s.charAt(j))) {
                set.add(s.charAt(j++));
                ans = Math.max(ans, j - i);
            } else {
                set.remove(s.charAt(i++));
            }
        }
        return ans;
    }

（2）優(yōu)化的滑動(dòng)窗口