title: Time complexity for getting the list of keys in a hash table?
body: <p>In a lot of languages you have the ability to get the list of keys from a hash table. Like the keySet() method for hash maps in java. How can this be obtained from a populated hash map? Isn't the hashfuction irreversible? Do you have too keep the keys in a separate list?</p>
  
  <p>So when i use functions to obtain the list of keyes used in a populated hash table what is the time complexity of that function?</p>
  
  <p>For my particular problem i happen to know the maximum number of entries in the hash table. Does this help?</p>