Implement an algorithm to determine if a string has all unique characters. CC 150 - Arrays and Strings

IsUniqueChars.java

package CC150.ArraysAndString;

import java.util.HashSet;
import java.util.Set;

public class IsUniqueChars {
	
	public static void main(String args[]) {
		System.out.println(isUniqueChars("abcdefghijklmnopqrstuvwxyz"));
	}
	
	/**
	 * Book's solution, just use boolean[] replaces Set, I need to understand unique characters!
	 * Then I could know I can create to boolean[256] to flag exist character.
	 * 
	 * In additon, no need to conver string to charArray,just use charAt(), save some space 
	 * 
	 * Conclusion: 1.understand unicode & ascII character.(only 256 characters) 2. study using boolean[] as flag
	 * In my solution, I just judge str == null and str.length() ==0 but not use str.length() >256
	 * 
	 */
	public static boolean isUniqueChars(String str) {
		if (str.length() > 256) {
			return false;
		}
		
		boolean[] flag = new boolean[256];
		char temp;
		for (int i=0; i<str.length(); i++) {
			temp = str.charAt(i);
			if(flag[temp]) {
				return false;
			} else {
				flag[temp] = true;
			}
		}
		return true;
	}
	
	/**
	 * Extra Data Structure Solution
	 * unique --> Set
	 */
	public static boolean isUniqueChars2(String str){
		if (str == null) {
			return false;
		}
		
		Set<Character> set = new HashSet<Character>();
		char[] arr = str.toCharArray();
		
		for (int i = 0; i < str.length(); i++) {
			if(set.contains(arr[i])) {
				return false;
			} else {
				set.add(arr[i]);
			}
		}
		
		return true;
	}
	
	/**
	 * brute force solution
	 * No extra data structure and space but time complexity increase   
	 */
	public static boolean isUniqueChars3(String str) {
		if (str == null) {
			return false;
		}
		char[] arr = str.toCharArray();
		for (int i=0; i<arr.length; i++) {
			for (int j=i+1; j<arr.length; j++) {
				if (arr[i] == arr[j]) {
					return false;
				}
			}
		}
		return true;
	}
}

The solution you have will work only for ASCII characters,
What about Strings which have 6000 unique characters.