Leetcode 804.Unique Morse Code Words （Java）

问题描述：

International Morse Code defines a standard encoding where each letter is mapped to a series of dots and dashes, as follows: "a"maps to ".-", "b" maps to "-...", "c" maps to "-.-.", and so on.

For convenience, the full table for the 26 letters of the English alphabet is given below:

[".-","-...","-.-.","-..",".","..-.","--.","....","..",".---","-.-",".-..","--","-.","---",".--.","--.-",".-.","...","-","..-","...-",".--","-..-","-.--","--.."]

Now, given a list of words, each word can be written as a concatenation of the Morse code of each letter. For example, "cab" can be written as "-.-.-....-", (which is the concatenation "-.-." + "-..." + ".-"). We'll call such a concatenation, the transformation of a word.

Return the number of different transformations among all words we have.

Example:
Input: words = ["gin", "zen", "gig", "msg"]
Output: 2
Explanation: 
The transformation of each word is:
"gin" -> "--...-."
"zen" -> "--...-."
"gig" -> "--...--."
"msg" -> "--...--."

There are 2 different transformations, "--...-." and "--...--.".

Note:

• The length of words will be at most 100.
• Each words[i] will have length in range [1, 12].

• words[i] will only consist of lowercase letters.

解决方案：

package test ;

import java.lang.* ;
import java.util.*;

class Solution {
    public int uniqueMorseRepresentations(String[] words) {
    	String[] MORSE = {".-","-...","-.-.","-..",".","..-.","--.","....","..",".---","-.-",".-..","--","-.","---",".--.","--.-",".-.","...","-","..-","...-",".--","-..-","-.--","--.."} ;
    	
    	Set<String> seen = new HashSet<String>() ;
    	
    	for(String word : words)
    	{
    		StringBuilder code = new StringBuilder() ;
    		for (char c : word.toCharArray())
    		{
    			code.append(MORSE[c - 'a']) ;
    		}
    		seen.add(code.toString()) ;
    	}
    	return seen.size() ;
    }
}

1. Java中的Set类：继承了Collection类，实现类有三个：

HashSet：按照哈希算法来存取集合中的对象，存取速度比较快。有三个特点：不能重复数据、可以出现空数据（null）、无序LinkedHashSet：在迭代访问Set中全部元素时，性能比HashSet好，但是插入时性能稍逊色于HashSet。与HashSet不同的一点是数据是有序的TreeSet：有三个特点：不能写入空数据（null）、有序、不能重复数据

2. 字符串：

• String：字符常量，不可改变，每调用String类中的方法时都会重新创建空间
• StringBuilder：单线程（线程不安全的），速度较快
• StringBuffer：多线程（线程安全的）
• 当需要对字符串进行重复操作时适合使用后两者

还是学习的过程，记录并前进！