Had a phone interview with the Ebay traffic team a few weeks back.
Here is the code question:
Find the longest word that are only consisted of elements from the periodic table in a dictionary.

/**
* sort all the words in the dictionary,
* and then validate from the longest word.
* once find one,
* @param dict
* @param elements
* @return
*/
private String findLongest(Set dict, Set elements) {
   PriorityQueue queue = new PriorityQueue(new Comparator() {
       @Override
       public int compare(String o1, String o2) {
           return o2.length() – o1.length();
       }
   }
}
while (!queue.isEmpty()) {
   String tmp = queue.poll();
if (isFormed(0, tmp, elements) {
   return tmp;
   }
}
return null;
}
private boolean isFormed(int i, String word, Set elements) {
   if (i == word.length()) {
   return true;
   }
   if (elements.contains(word.substring(i, i + 1)))
      if (isFormed(i + 1, word, elements))
      return true;
      if (i < word.length() – 1) { 
         if (elements.contains(word.substring(i, i + 2))) 
         if (isFormed(i + 2, word, elements)) 
         return true; 
       }  
   return false; 
 }