[CC150][1.1] Implement an algorithm to determine if a string has all unique characters. What if you cannot use additional data structures? (not applicable for Unicode)

UniqueString.java

import java.util.Scanner;
public class UniqueString {
	
	public static boolean unique(String s) {
		if (s == null) return false;
		boolean[] flag = new boolean[256];
		for (int i = 0; i < s.length(); i++) {
			if (flag[s.charAt(i)]) 
				return false;
			else
				flag[s.charAt(i)] = true;
		}
		return true;
	}

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner in = new Scanner(System.in);
		while (in.hasNextLine()) {
			String s = in.nextLine();
			if (unique(s))
				System.out.println("Unique");
			else
				System.out.println("Not Unique") ;
		}
		in.close();
	}
}

1. You should return the result directly when the length of string is 0 or bigger than 256.