[LeetCode] Encode and Decode TinyURL 編碼和解碼精簡URL地址
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.
這道題讓我們編碼和解碼精簡URL地址,這其實很有用,因為有的連結地址特別的長,就很煩,如果能精簡成固定的長度,就很清爽。最簡單的一種編碼就是用個計數器,當前是第幾個存入的url就編碼成幾,然後解碼的時候也能根據數字來找到原來的url,參見程式碼如下:
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; };
上面這種方法雖然簡單,但是缺點卻很多,首先,如果接受到多次同一url地址,仍然會當做不同的url來處理。當然這個缺點可以通過將vector換成雜湊表,每次先查詢url是否已經存在。雖然這個缺點可以克服掉,但是由於是用計數器編碼,那麼當前伺服器存了多少url就曝露出來了,也許會有安全隱患。而且計數器編碼另一個缺點就是數字會不斷的增大,那麼編碼的長度也就不是確定的了。而題目中明確推薦了使用六位隨機字元來編碼,那麼我們只要在所有大小寫字母和數字中隨機產生6個字元就可以了,我們用雜湊表建立6位字元和url之間的對映,如果隨機生成的字元之前已經存在了,我們就繼續隨機生成新的字串,直到生成了之前沒有的字串為止。下面的程式碼中使用了兩個雜湊表,目的是為了建立六位隨機字串和url之間的相互對映,這樣進來大量的相同url時,就不用生成新的隨機字串了。當然,不加這個功能也能通過OJ,這道題的OJ基本上是形同虛設,兩個函式分別直接返回引數字串也能通過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; 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; };
參考資料: