# Design Tiny URL

Given a long url, make it shorter. To make it simpler, let's ignore the domain name.

You should implement two methods:

* `longToShort(url)`. Convert a long url to a short url.
* `shortToLong(url)`. Convert a short url to a long url starts with `http://tiny.url/`.

You can design any shorten algorithm, the judge only cares about two things:

1. The `short key`'s length should `equal to 6`(without domain and slash). And the acceptable characters are`[a-zA-Z0-9]`. For example:`abcD9E`
2. No two long urls mapping to the same short url and no two short urls mapping to the same long url.

## Example

Given url =`http://www.lintcode.com/faq/?id=10`, run the following code (or something similar):

```
short_url = longToShort(url) // may return http://tiny.url/abcD9E
long_url = shortToLong(short_url) // return http://www.lintcode.com/faq/?id=10
```

The short\_url you return should be unique short url and start with`http://tiny.url/`and`6`acceptable characters. For example "<http://tiny.url/abcD9E>" or something else.

The long\_url should be`http://www.lintcode.com/faq/?id=10`in this case.

## Note

两种方法，随机或者base62

都要建立long和short的映射关系

## Code

```java
// version 1: Random
public class TinyUrl {
    private Map<String, String> long2Short;
    private Map<String, String> short2Long;

    public TinyUrl() {
        long2Short = new HashMap<String, String>();
        short2Long = new HashMap<String, String>();
    }

    /**
     * @param url a long url
     * @return a short url starts with http://tiny.url/
     */
    public String longToShort(String url) {
        if (long2Short.containsKey(url)) {
            return long2Short.get(url);
        }

        while (true) {
            String shortURL = generateShortURL();
            if (!short2Long.containsKey(shortURL)) {
                short2Long.put(shortURL, url);
                long2Short.put(url, shortURL);
                return shortURL;
            }
        }
    }

    /**
     * @param url a short url starts with http://tiny.url/
     * @return a long url
     */
    public String shortToLong(String url) {
        if (!short2Long.containsKey(url)) {
            return null;
        }

        return short2Long.get(url);
    }

    private String generateShortURL() {
        String allowedChars = 
            "0123456789" +
            "abcdefghijklmnopqrstuvwxyz" + 
            "ABCDEFGHIJKLMNOPQRSTUVWXYZ";

        Random rand = new Random();
        String shortURL = "http://tiny.url/";
        for (int i = 0; i < 6; i++) {
            int index = rand.nextInt(62);
            shortURL += allowedChars.charAt(index);
        }

        return shortURL;
    }
}
```

```java
/ version 2: base62
public class TinyUrl {
    public static int GLOBAL_ID = 0;
    private HashMap<Integer, String> id2url = new HashMap<Integer, String>();
    private HashMap<String, Integer> url2id = new HashMap<String, Integer>();

    private String getShortKey(String url) {
        return url.substring("http://tiny.url/".length());
    }

    private int toBase62(char c) {
        if (c >= '0' && c <= '9') {
            return c - '0';
        }
        if (c >= 'a' && c <= 'z') {
            return c - 'a' + 10;
        }
        return c - 'A' + 36;
    }

    private int shortKeytoID(String short_key) {
        int id = 0;
        for (int i = 0; i < short_key.length(); ++i) {
            id = id * 62 + toBase62(short_key.charAt(i));
        }
        return id;
    }

    private String idToShortKey(int id) {
        String chars = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
        String short_url = "";
        while (id > 0) {
            short_url = chars.charAt(id % 62) + short_url;
            id = id / 62;
        }
        while (short_url.length() < 6) {
            short_url = "0" + short_url;
        }
        return short_url;
    }

    /**
     * @param url a long url
     * @return a short url starts with http://tiny.url/
     */
    public String longToShort(String url) {
        if (url2id.containsKey(url)) {
            return "http://tiny.url/" + idToShortKey(url2id.get(url));
        }
        GLOBAL_ID++;
        url2id.put(url, GLOBAL_ID);
        id2url.put(GLOBAL_ID, url);
        return "http://tiny.url/" + idToShortKey(GLOBAL_ID);
    }

    /**
     * @param url a short url starts with http://tiny.url/
     * @return a long url
     */
    public String shortToLong(String url) {
        String short_key = getShortKey(url);
        int id = shortKeytoID(short_key);
        return id2url.get(id);
    }
}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://luj.gitbook.io/code/designood/design-tiny-url.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.