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

1 class Solution
2 {
3 public:
4     // Encodes a URL to a shortened URL.
5     string encode(string longUrl) { return longUrl; }
6     // Decodes a shortened URL to its original URL.
7     string decode(string shortUrl) { return shortUrl; }
8 };

1 class Solution
2 {
3 public:
4     string theLatestUrl;
5     string encode(string longUrl) { theLatestUrl = longUrl; return "whatever, doesn't matter"; }
6     string decode(string shortUrl) { return theLatestUrl; }
7 };

● 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。

 1 class Solution
 2 {
 3 public:
 4     unordered_map<string, string> um;    
 5     string encode(string longUrl)
 6     {        
 7         const string baseUrl = "http://tinyurl.com/";
 8         int r;
 9         string ran;
10         srand(1); 
11         for (r = rand() % 100000, ran = to_string(r); um.count(baseUrl + ran) == 1; r = rand() % 100000, ran = to_string(r));
12             // 随机取一个空的位置把网址放进去
13         um[baseUrl + ran] = longUrl;
14         return baseUrl + ran;
15     }    
16     string decode(string shortUrl)
17     {
18         if (um.count(shortUrl) == 1)
19             return um[shortUrl];
20         return "";
21     }
22 };