https://www.lintcode.com/problem/157/

# Description

Implement an algorithm to determine if a string has all unique characters.

# Example

Example 1:

Input: "abc_____"
Output: false

Example 2:

Input:  "abc"
Output: true

Challenge

What if you can not use extra space?

HashSet - O(n) Space

	public class Solution {
	/**
	* @param str: A string
	* @return: a boolean
	*/
	public boolean isUnique(String str) {
	Set<Character> set = new HashSet<>();
	for (int i = 0; i < str.length(); i++) {
	if (set.contains(str.charAt(i))) {
	return false;
	} else {
	set.add(str.charAt(i));
	}

	}
	return true;
	}
	}

使用 map 記錄位置 O (1) space

	public class Solution {
	/**
	* @param str: A string
	* @return: a boolean
	*/
	public boolean isUnique(String str) {
	boolean[] word = new boolean[256];
	for (int i = 0; i < str.length(); i++) {
	if (word[str.charAt(i)]) return false;
	word[str.charAt(i)] = true;
	}
	return true;
	}
	}