查詢字串中第一個非重複字元的3種方法

<b>import</b> java.io.IOException;
<b>import</b> java.util.ArrayList;
<b>import</b> java.util.HashMap;
<b>import</b> java.util.HashSet;
<b>import</b> java.util.LinkedHashMap;
<b>import</b> java.util.List;
<b>import</b> java.util.Map;
<b>import</b> java.util.Map.Entry;
<b>import</b> java.util.Set;

<font><i>/**
 * Java Program to find first duplicate, non-repeated character in a String.
 * It demonstrate three simple example to do this programming problem.
 *
 * @author Javarevisited
 */</i></font><font>
<b>public</b> <b>class</b> Programming {

</font><font><i>/*
* Using LinkedHashMap to find first non repeated character of String
* Algorithm :
*Step 1: get character array and loop through it to build a 
*hash table with char and their count.
*Step 2: loop through LinkedHashMap to find an entry with 
*value 1, that's your first non-repeated character,
*as LinkedHashMap maintains insertion order.
*/</i></font><font>
<b>public</b> <b>static</b> <b>char</b> getFirstNonRepeatedChar(String str) {
Map<Character,Integer> counts = <b>new</b> LinkedHashMap<>(str.length());

<b>for</b> (<b>char</b> c : str.toCharArray()) {
counts.put(c, counts.containsKey(c) ? counts.get(c) + 1 : 1);
}

<b>for</b> (Entry<Character,Integer> entry : counts.entrySet()) {
<b>if</b> (entry.getValue() == 1) {
<b>return</b> entry.getKey();
}
}
<b>throw</b> <b>new</b> RuntimeException(</font><font>"didn't find any non repeated Character"</font><font>);
}


</font><font><i>/*
* Finds first non repeated character in a String in just one pass.
* It uses two storage to cut down one iteration, standard space vs time
* trade-off.Since we store repeated and non-repeated character separately,
* at the end of iteration, first element from List is our first non
* repeated character from String.
*/</i></font><font>
<b>public</b> <b>static</b> <b>char</b> firstNonRepeatingChar(String word) {
Set<Character> repeating = <b>new</b> HashSet<>();
List<Character> nonRepeating = <b>new</b> ArrayList<>();
<b>for</b> (<b>int</b> i = 0; i < word.length(); i++) {
<b>char</b> letter = word.<b>char</b>At(i);
<b>if</b> (repeating.contains(letter)) {
<b>continue</b>;
}
<b>if</b> (nonRepeating.contains(letter)) {
nonRepeating.remove((Character) letter);
repeating.add(letter);
} <b>else</b> {
nonRepeating.add(letter);
}
}
<b>return</b> nonRepeating.get(0);
}


</font><font><i>/*
* Using HashMap to find first non-repeated character from String in Java.
* Algorithm :
* Step 1 : Scan String and store count of each character in HashMap
* Step 2 : traverse String and get count for each character from Map.
*Since we are going through String from first to last character,
*when count for any character is 1, we break, it's the first
*non repeated character. Here order is achieved by going
*through String again.
*/</i></font><font>
<b>public</b> <b>static</b> <b>char</b> firstNonRepeatedCharacter(String word) {
HashMap<Character,Integer> scoreboard = <b>new</b> HashMap<>();
</font><font><i>// build table [char -> count]</i></font><font>
<b>for</b> (<b>int</b> i = 0; i < word.length(); i++) {
<b>char</b> c = word.<b>char</b>At(i);
<b>if</b> (scoreboard.containsKey(c)) {
scoreboard.put(c, scoreboard.get(c) + 1);
} <b>else</b> {
scoreboard.put(c, 1);
}
}
</font><font><i>// since HashMap doesn't maintain order, going through string again</i></font><font>
<b>for</b> (<b>int</b> i = 0; i < word.length(); i++) {
<b>char</b> c = word.<b>char</b>At(i);
<b>if</b> (scoreboard.get(c) == 1) {
<b>return</b> c;
}
}
<b>throw</b> <b>new</b> RuntimeException(</font><font>"Undefined behaviour"</font><font>);
}

}
</font>

<b>import</b> <b>static</b> org.junit.Assert.*;
<b>import</b> org.junit.Test;

<b>public</b> <b>class</b> ProgrammingTest {

@Test
<b>public</b> <b>void</b> testFirstNonRepeatedCharacter() {
assertEquals('b', Programming.firstNonRepeatedCharacter(<font>"abcdefghija"</font><font>));
assertEquals('h', Programming.firstNonRepeatedCharacter(</font><font>"hello"</font><font>));
assertEquals('J', Programming.firstNonRepeatedCharacter(</font><font>"Java"</font><font>));
assertEquals('i', Programming.firstNonRepeatedCharacter(</font><font>"simplest"</font><font>));
}

@Test
<b>public</b> <b>void</b> testFirstNonRepeatingChar() {
assertEquals('b', Programming.firstNonRepeatingChar(</font><font>"abcdefghija"</font><font>));
assertEquals('h', Programming.firstNonRepeatingChar(</font><font>"hello"</font><font>));
assertEquals('J', Programming.firstNonRepeatingChar(</font><font>"Java"</font><font>));
assertEquals('i', Programming.firstNonRepeatingChar(</font><font>"simplest"</font><font>));
}

@Test
<b>public</b> <b>void</b> testGetFirstNonRepeatedChar() {
assertEquals('b', Programming.getFirstNonRepeatedChar(</font><font>"abcdefghija"</font><font>));
assertEquals('h', Programming.getFirstNonRepeatedChar(</font><font>"hello"</font><font>));
assertEquals('J', Programming.getFirstNonRepeatedChar(</font><font>"Java"</font><font>));
assertEquals('i', Programming.getFirstNonRepeatedChar(</font><font>"simplest"</font><font>));
}
}
</font>