有三种方法可以找到第一个非重复字符。每个都使用自己的算法来完成这个编程任务。
第一种算法在getFirstNonRepeatedChar（String str）方法中实现。它首先从给定的String获取字符数组并循环遍历它，以构建一个哈希表，其中字符为键，其计数为值。在下一步中，它遍历LinkedHashMap以查找值为1的条目，这是您的第一个非重复字符，因为LinkedHashMap维护插入顺序，并且我们从头到尾迭代字符数组。不好的部分是它需要两次迭代，第一次与String中的字符数成比例，第二次与String中的重复字符数成比例。在最坏的情况下，String最后包含非重复字符，解决此问题需要2 * N时间。
第二种方法是在firstNonRepeatingChar（String word）上编码，该解决方案只在一次传递中找到String中的第一个非重复字符。它应用了经典的时空权衡技术。使用两个存储来减少一次迭代，标准空间与时间的权衡。由于我们分别存储重复和非重复的字符，因此在迭代结束时，List中的第一个元素是String中的第一个非重复字符。这个稍微好于前一个，但如果String中没有非重复字符，则选择返回null或空字符串。
第三种方法是在firstNonRepeatedCharacter（String word）方法中实现的。它与第一个非常相似，只是除了LinkedHashMap之外，我们使用了HashMap。由于以后不保证任何顺序，我们必须依靠原始字符串来查找第一个非重复字符。这是第三种解决方案的算法。第一步：扫描字符串并在HashMap中存储每个字符的计数。第二步：遍历String并从Map获取每个字符的计数。由于我们要从第一个字符到最后一个字符遍历字符串，当任何字符的计数为1时，我们将中断，这是第一个非重复字符。这里的顺序是通过再次遍历字符串来实现的。
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;
/**
* Java Program to find first duplicate, non-repeated character in a String.
* It demonstrate three simple example to do this programming problem.
*
* @author Javarevisited
*/
public class Programming {
/*
* 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.
*/
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");
}
/*
* 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.
*/
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);
}
/*
* 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.
*/
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");
}
}
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查找第一个唯一字符的JUnit测试
下面是一些JUnit测试案例来测试每个方法。我们测试不同类型的输入，一个包含重复项，另一个不包含重复项。由于程序没有定义空字符串、空字符串的情况下要做什么，以及如果只包含重复项，则返回什么，所以您可以选用有意义的方式来做。
import static org.junit.Assert.*;
import org.junit.Test;
public class ProgrammingTest {
@Test
public void testFirstNonRepeatedCharacter() {
assertEquals('b', Programming.firstNonRepeatedCharacter("abcdefghija"));
assertEquals('h', Programming.firstNonRepeatedCharacter("hello"));
assertEquals('J', Programming.firstNonRepeatedCharacter("Java"));
assertEquals('i', Programming.firstNonRepeatedCharacter("simplest"));
}
@Test
public void testFirstNonRepeatingChar() {
assertEquals('b', Programming.firstNonRepeatingChar("abcdefghija"));
assertEquals('h', Programming.firstNonRepeatingChar("hello"));
assertEquals('J', Programming.firstNonRepeatingChar("Java"));
assertEquals('i', Programming.firstNonRepeatingChar("simplest"));
}
@Test
public void testGetFirstNonRepeatedChar() {
assertEquals('b', Programming.getFirstNonRepeatedChar("abcdefghija"));
assertEquals('h', Programming.getFirstNonRepeatedChar("hello"));
assertEquals('J', Programming.getFirstNonRepeatedChar("Java"));
assertEquals('i', Programming.getFirstNonRepeatedChar("simplest"));
}
}
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这就是如何在Java中找到String的第一个非重复字符。我们已经看到了解决这个问题的三种方法，虽然它们使用了非常相似的逻辑，但它们彼此不同。这个程序也非常适合初学者掌握Java Collection框架。它使您有机会探索不同的Map实现，并了解HashMap和LinkedHashMap之间的区别，以决定何时使用它们。