535. Encode and Decode TinyURL 

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import java.util.HashMap;
import java.util.Map;
import java.util.concurrent.ThreadLocalRandom;

public class Codec {

    private static final String DICT = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789";
    private static final int SHORT_LENGTH = 6;

    private final Map<String, String> longToShortMap = new HashMap<>();
    private final Map<String, String> shortToLongMap = new HashMap<>();


    // Encodes a URL to a shortened URL.
    public String encode(String longUrl) {
        String shortUrl = null;
        while (!longToShortMap.containsKey(longUrl)) {
            char[] chars = new char[SHORT_LENGTH];
            for (int i = 0; i < chars.length; i++) {
                chars[i] = DICT.charAt(ThreadLocalRandom.current().nextInt(DICT.length()));
            }
            shortUrl = new String(chars);
            if (shortToLongMap.containsKey(shortUrl)) {
                continue;
            }
            longToShortMap.put(longUrl, shortUrl);
            shortToLongMap.put(shortUrl, longUrl);
        }

        return shortUrl;
    }

    // Decodes a shortened URL to its original URL.
    public String decode(String shortUrl) {
        return shortToLongMap.get(shortUrl);
    }

}

References

535. Encode and Decode TinyURL