題目:給定一個string,找出第一個string裏的unique character,如“cisco”的話就應該是i,“cat”的話就是c。
第一種方法是用LinkedHashMap保留數據的順序,過一遍string,對字符進行計數,然後找到第一個數量爲1的字符即可。
第二種方法是用一個HashSet來存已經重複的字符,然後用個arraylist來存僅出現了一次的字符。
第三種方法是用HashMap來存數據以及count,然後再走一遍string,對每個字符檢查計數是否爲1.
代碼:
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