Leetcode 535. Encode and Decode 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.
这个简单，设置一个从0开始的数字（类似于数据库中的key），然后一一映射到这些长url中就好了，再把数字作为短url。代码如下：

public class Encode_and_Decode_TinyURL_535 {
ArrayList<String> urLs=new ArrayList<String>();

// Encodes a URL to a shortened URL.
public String encode(String longUrl) {
int index=urLs.size();
urLs.add(longUrl);
return String.valueOf(index);
}

// Decodes a shortened URL to its original URL.
public String decode(String shortUrl) {
int index=Integer.parseInt(shortUrl);
return urLs.get(index);
}
}

但是leetcode上的牛人提出了几点问题：

如果我多次将同一个长url来进行编码，那么我会得到好几个短url，这浪费了存储空间。
会将多少个url已被编码暴露在阳光下，有些人也许会为了得到特殊的数字故意多次输入相同的长url。
只使用数字意味着短url也许会变得特别长。长度为6（100000）也只能获得百万级别的编码数。如果采取数字或大小写字母组合的6个字符的话，能提供（10+26*2）*6=56,800,235,584 个编码，比百万级可观多了！

下面的解题思路不再是只用字母，而是保证短url长度为6，但是里面可以包含数字或大小写字母。比如http://tinyurl.com/KtLa2U这种格式（貌似新浪微博就是这样转换长url的？）当一个长url已经被转化成短url了，将会直接提取之前转化成的短url，保证存储空间不被浪费。

public class Codec {
Map<String, String> index = new HashMap<String, String>();
Map<String, String> revIndex = new HashMap<String, String>();
static String BASE_HOST = "http://tinyurl.com/";

// Encodes a URL to a shortened URL.
public String encode(String longUrl) {
if (revIndex.containsKey(longUrl)) return BASE_HOST + revIndex.get(longUrl);
String charSet = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
String key = null;
do {
StringBuilder sb = new StringBuilder();
for (int i = 0; i < 6; i++) {
int r = (int) (Math.random() * charSet.length());
sb.append(charSet.charAt(r));
}
key = sb.toString();
} while (index.containsKey(key));
index.put(key, longUrl);
revIndex.put(longUrl, key);
return BASE_HOST + key;
}

// Decodes a shortened URL to its original URL.
public String decode(String shortUrl) {
return index.get(shortUrl.replace(BASE_HOST, ""));
}
}