Note: This is a companion problem to the System Design problem: Design TinyURL.
TinyURL is a URL shortening service where you enter a URL such as https://leetcode.com/problems/design-tinyurl and it returns a short URL such as http://tinyurl.com/4e9iAk.
Design the encode and decode methods for the TinyURL service. There is no restriction on how your encode/decode algorithm should work. You just need to ensure that a URL can be encoded to a tiny URL and the tiny URL can be decoded to the original URL.
这道题让我们编码和解码精简URL地址，这其实很有用，因为有的链接地址特别的长，就很烦，如果能精简成固定的长度，就很清爽。最简单的一种编码就是用个计数器，当前是第几个存入的url就编码成几，然后解码的时候也能根据数字来找到原来的url，参见代码如下：
解法一：
class Solution { public: // Encodes a URL to a shortened URL. string encode(string longUrl) { url.push_back(longUrl); return "http://tinyurl.com/" + to_string(url.size() - 1); } // Decodes a shortened URL to its original URL. string decode(string shortUrl) { auto pos = shortUrl.find_last_of("/"); return url[stoi(shortUrl.substr(pos + 1))]; } private: vector<string> url; };
上面这种方法虽然简单，但是缺点却很多，首先，如果接受到多次同一url地址，仍然会当错不同的url来处理。当然这个缺点可以通过将vector换成哈希表，每次先查找url是否已经存在。虽然这个缺点可以克服掉，但是由于是用计数器编码，那么当前服务器存了多少url就曝露出来了，也许会有安全隐患。而且计数器编码另一个缺点就是数字会不断的增大，那么编码的长度也就不是确定的了。而题目中明确推荐了使用六位随机字符来编码，那么我们只要在所有大小写字母和数字中随机产生6个字符就可以了，我们用哈希表建立6位字符和url之间的映射，如果随机生成的字符之前已经存在了，我们就继续随机生成新的字符串，直到生成了之前没有的字符串为止。下面的代码中使用了两个哈希表，目的是为了建立六位随机字符串和url之间的相互映射，这样进来大量的相同url时，就不用生成新的随机字符串了。当然，不加这个功能也能通过OJ，这道题的OJ基本上是形同虚设，两个函数分别直接返回参数字符串也能通过OJ，囧～
解法二：
class Solution { public: Solution() { dict = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"; short2long.clear(); long2short.clear(); srand(time(NULL)); } // Encodes a URL to a shortened URL. string encode(string longUrl) { if (long2short.count(longUrl)) { return "http://tinyurl.com/" + long2short[longUrl]; } int idx = 0, val = 0; string randStr; for (int i = 0; i < 6; ++i) randStr.push_back(dict[rand() % 62]); while (short2long.count(randStr)) { randStr[idx] = dict[rand() % 62]; idx = (idx + 1) % 5; } short2long[randStr] = longUrl; long2short[longUrl] = randStr; return "http://tinyurl.com/" + randStr; } // Decodes a shortened URL to its original URL. string decode(string shortUrl) { string randStr = shortUrl.substr(shortUrl.find_last_of("/") + 1); return short2long.count(randStr) ? short2long[randStr] : shortUrl; } private: unordered_map<string, string> short2long, long2short; string dict; };
参考资料：
https://discuss.leetcode.com/topic/81637/two-solutions-and-thoughts/2
https://discuss.leetcode.com/topic/81736/c-solution-using-random-just-for-fun
LeetCode All in One 题目讲解汇总(持续更新中...)
[LeetCode] Encode and Decode TinyURL 编码和解码小URL地址
原文：http://www.cnblogs.com/grandyang/p/6562209.html