找出一個字符串中最長不重複子串的長度

	public static int lengthOfLongestSubstring(String str) {
        int n = str.length();
        int ans = 0;
        for (int i = 0; i < n; i++) {
            for (int j = i + 1; j <= n; j++) {
                if (allUnique(str, i, j)) {
                    ans = Math.max(ans, j - i);
                }
            }
        }
        return ans;
    }

    public static boolean allUnique(String s, int start, int end) {
        Set<Character> set = new HashSet<>();
        for (int i = start; i < end; i++) {
            Character ch = s.charAt(i);
            if (set.contains(ch)) {
                return false;
            }
            set.add(ch);
        }
        return true;
    }