535. Encode and Decode TinyURL

▶ 要求给出一种对 URL 网址进行压缩和解压的算法，例如 https://leetcode.com/problems/design-tinyurl ←→ http://tinyurl.com/4e9iAk，在不允许打表的情况下应该无法压缩的。

● 代码，6 ms，完全不压缩，just for fun

class Solution
{
public:
    // Encodes a URL to a shortened URL.
    string encode(string longUrl) { return longUrl; }
    // Decodes a shortened URL to its original URL.
    string decode(string shortUrl) { return shortUrl; }
};

class Solution
{
public:
    string theLatestUrl;
    string encode(string longUrl) { theLatestUrl = longUrl; return "whatever, doesn't matter"; }
    string decode(string shortUrl) { return theLatestUrl; }
};

● 代码，9 ms，随机数打表法，加密之后的字符串只与原网址字符串在表中的存放位置有关，与其内容无关。类似的有各种打表法，需要依赖散列表来解码，时间复杂度 O(1)，空间开销大。

class Solution
{
public:
    unordered_map<string, string> um;    
    string encode(string longUrl)
    {        
        const string baseUrl = "http://tinyurl.com/";
        int r;
        string ran;
        srand(1); 
        for (r = rand() % 100000, ran = to_string(r); um.count(baseUrl + ran) == 1; r = rand() % 100000, ran = to_string(r));
            // 随机取一个空的位置把网址放进去
        um[baseUrl + ran] = longUrl;
        return baseUrl + ran;
    }    
    string decode(string shortUrl)
    {
        if (um.count(shortUrl) == 1)
            return um[shortUrl];
        return "";
    }
};