在字符串中查找第一个非重复字符的3种方法

<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>