first unique character in a string

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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 againfor (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

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