# Encode and Decode TinyURL

Note: This is a companion problem to the

> [System Design](https://leetcode.com/discuss/interview-question/system-design/) problem: [Design TinyURL](https://leetcode.com/discuss/interview-question/124658/Design-a-URL-Shortener-\(-TinyURL-\)-System/) .

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.

分析

1. string.ascii\_letters+string.digits 提供了可选的字符
2. ''.join(random.choice(chars) for \_ in range(6))得到了code
3. 2个map code2url 和url2code
4. encode时候有个while longURL not in url2code 保证了code重复情况下依然可以create新code

```
class Codec:
    alphabet = string.ascii_letters+string.digits
    code_base = "http://tinyurl.com/"

    def __init__(self):
        self.url2code = {}
        self.code2url = {}

    def encode(self, longUrl):
        """Encodes a URL to a shortened URL.

        :type longUrl: str
        :rtype: str
        """
        while longUrl not in self.url2code:
            code = ''.join(random.choice(self.alphabet) for _ in range(6))
            if code not in self.code2url:
                self.code2url[code] = longUrl
                self.url2code[longUrl] = code
        return self.code_base + code


    def decode(self, shortUrl):
        """Decodes a shortened URL to its original URL.

        :type shortUrl: str
        :rtype: str
        """
        return self.code2url[shortUrl[-6:]] 

# Your Codec object will be instantiated and called as such:
# codec = Codec()
# codec.decode(codec.encode(url))
```