题目：给定一个string，找出第一个string里的unique character，如“cisco”的话就应该是i，“cat”的话就是c。

第一种方法是用LinkedHashMap保留数据的顺序，过一遍string，对字符进行计数，然后找到第一个数量为1的字符即可。

第二种方法是用一个HashSet来存已经重复的字符，然后用个arraylist来存仅出现了一次的字符。

第三种方法是用HashMap来存数据以及count，然后再走一遍string，对每个字符检查计数是否为1.

代码：

import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;

public class FirstUniqueCharacter {
    //方法一
	public static char getFirstNonRepeatedChar(String str) {
		Map<Character, Integer> counts = new LinkedHashMap<>(str.length());
		for (char c : str.toCharArray()) {
			counts.put(c, counts.containsKey(c) ? counts.get(c) + 1 : 1);
		}
		for (Entry<Character, Integer> entry : counts.entrySet()) {
			if (entry.getValue() == 1) {
				return entry.getKey();
			}
		}
		throw new RuntimeException("didn't find any non repeated Character");
	}
   
	//方法二
	public static char firstNonRepeatingChar(String word) {
		Set<Character> repeating = new HashSet<>();
		List<Character> nonRepeating = new ArrayList<>();
		for (int i = 0; i < word.length(); i++) {
			char letter = word.charAt(i);
			if (repeating.contains(letter)) {
				continue;
			}
			if (nonRepeating.contains(letter)) {
				nonRepeating.remove((Character) letter);
				repeating.add(letter);
			} else {
				nonRepeating.add(letter);
			}
		}
		return nonRepeating.get(0);
	}

	//方法三
	public static char firstNonRepeatedCharacter(String word) {
		HashMap<Character, Integer> scoreboard = new HashMap<>();
		// build table [char -> count]
		for (int i = 0; i < word.length(); i++) {
			char c = word.charAt(i);
			if (scoreboard.containsKey(c)) {
				scoreboard.put(c, scoreboard.get(c) + 1);
			} else {
				scoreboard.put(c, 1);
			}
		} // since HashMap doesn't maintain order, going through string again
		for (int i = 0; i < word.length(); i++) {
			char c = word.charAt(i);
			if (scoreboard.get(c) == 1) {
				return c;
			}
		}
		throw new RuntimeException("Undefined behaviour");
	}

}

Reference：http://javarevisited.blogspot.com/2014/03/3-ways-to-find-first-non-repeated-character-String-programming-problem.html

first unique character in a string