String:判断字符串是否有重复

public static boolean test(String string) {
        if (string.length() > 256) {
            return false;
        }
        boolean[] charSet = new boolean[256];

        for (int i = 0; i < string.length(); i++) {
            int val = string.charAt(i);
            if (charSet[val]) {
                return false;
            }
            charSet[val] = true;

        }
        return true;
    }