title: Find Kth largest number in single pass without storing the whole array
body: <p>The algo I have in mind is  </p>
  
  <ul>
  <li>keep an MaxHeap of size K</li>
  <li>insert each element</li>
  <li>drop out smaller value if heap is full</li>
  <li>In the end, Kth max is the smaller of MaxHeap </li>
  </ul>
  
  <p>That will give me O(NlogK). Is there a better algorithm? I can't do quick selection, because the array can't be stored in memory.</p>