[Leetcode] 535. Encode and Decode TinyURL 解题报告
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题目:
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.
思路:
目测这道题目应该有多种不同的解法,这里给出其中的一种。维护一个全局的id,每次遇到新的longUrl,就给它赋一个新的id。现在问题就是如何把id映射为一个shortUrl了。我们采用的做法是:把这个shortUrl看作是一个“62进制”的数,其基由10个数字加上26个小写字母以及26个大写字母组成。这样问题就转化为10进制的id和62进制的shortUrl了(用string表示)之间的转换了。
encode:遇到一个新的longUrl后,我们赋给它一个新的id,然后将这个10进制的id转换为62进制的shortUrl,并且建立longUrl到shortUrl之间的哈希映射,以及id到longUrl之间的映射,以便于decode的时候使用。
decode:遇到一个shortUrl之后,我们将它转换为10进制的id,然后在哈希表中查找,返回对应的longUrl即可。
当然也可以直接建立shortUrl到longUrl之间的映射,而不是id到longUrl之间的映射。这样在decode的时候,就不需要进行进制之间的转换了。读者可以试着实现一下。
代码:
class Solution {public: // Encodes a URL to a shortened URL. string encode(string longUrl) { if(long_short.find(longUrl) != long_short.end()) { return long_short[longUrl]; } string res = ""; id++; int count = id; while(count > 0) { res = dict[count % 62] + res; count /= 62; } while(res.size() < 6) { res = "0" + res; } long_short[longUrl] = res; id_long[id] = longUrl; return res; } // Decodes a shortened URL to its original URL. string decode(string shortUrl) { int id = 0; for(int i = 0; i < shortUrl.size(); i++) { id = 62 * id + (int)(dict.find(shortUrl[i])); } if(id_long.find(id) != id_long.end()) { return id_long[id]; } return ""; }private: string dict = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ"; int id = 0; unordered_map<string,string> long_short; // key is longURL, value is shortURL unordered_map<int, string> id_long; // key is id in DB, value is longURL};// Your Solution object will be instantiated and called as such:// Solution solution;// solution.decode(solution.encode(url));
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