804. Unique Morse Code Words

算法 
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 "--...--.".

这个题目是说把一个数组的单词转换成morse密码，输出不重复的morse密码个数。

思路是先遍历单词数组，根据给定的morse密码数组把单词转换成morse字符串；使用一个数组来存放每次转换后的字符串，在每次加入数组前先比较是否已经有相同的字符串，如果没有则计数器加1。

C++实现:

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class Solution {
public:
    int uniqueMorseRepresentations(vector<string>& words) {
        vector<string> morse;
        int result = 0;
        morse = {".-","-...","-.-.","-..",".","..-.","--.","....","..",".---","-.-",".-..","--","-.","---",".--.","--.-",".-.","...","-","..-","...-",".--","-..-","-.--","--.."};
         
        vector<string> morsewords;
        //遍历words,转换为morse，比较计数
        for(string word:words){
            string ctomorse = "";
            bool flag = true;
            for(char c:word){
                ctomorse += morse[c-'a'];   //word转化为morse
            }
            //先判断是否存在
            for(string wordmorse:morsewords){
                if(ctomorse==wordmorse){
                    flag = false;
                    break;
                }else{
                    continue;
                }
            }
            if(flag){
                result += 1;
            }
            //后加入数组
            morsewords.push_back(ctomorse);     //注意一下这里使用push_back()
        }
        return result;
    }
};

需要注意的地方是vector的使用。一开始使用morsewords[i]这样的数组的方法加入vector，这样不能动态地改变vector的长度，一旦初始化确定长度之后又会因为长度固定了结果不对。所以需要使用push_back()函数动态地加入，就不需要初始化指定长度。

Javascript实现:

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/**
 * @param {string[]} words
 * @return {number}
 */
var uniqueMorseRepresentations = function(words) {
    var morse = [".-","-...","-.-.","-..",".","..-.","--.","....","..",".---","-.-",".-..","--","-.","---",".--.","--.-",".-.","...","-","..-","...-",".--","-..-","-.--","--.."];
    var count = 0;
    var morsewords = [];
    //遍历words
    for(let word of words){
        var morseword = '';
        var a = 'a';
 
        var flag = true;
        for(let c of word){
            morseword += morse[c.charCodeAt(0)-a.charCodeAt(0)];  //每个word转换为morseword
        }
         
        for(let m of morsewords){
            if(morseword == m){
                flag = false;
                break;
            }else {
                continue
            }
        }
        if(flag){
            count +=1;
        }
        morsewords.push(morseword);
    }
    return count;
};

用js实现的时候在ASCII码转换的时候不能直接像C++那样字符的加减，需要使用charCodeAt()函数，将字符串的第几个字符转换成ACII码。

贴一下解答里面的js的实现方法，学习一下。

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var uniqueMorseRepresentations = function(words) {
    const mapping = [".-","-...","-.-.","-..",".","..-.","--.","....","..",".---","-.-",".-..","--","-.","---",".--.","--.-",".-.","...","-","..-","...-",".--","-..-","-.--","--.."];
    const charToIndex = char => char.charCodeAt(0) - 97;
    return new Set(words.map(word => word.split('').reduce((morseCode, cur) => morseCode + mapping[charToIndex(cur)], ''))).size;
};