title: Find the last element inserted into the min heap?
body: <p>I am looking at this challenge:</p>
  <blockquote>
  <p>Consider the min-heap [15, 27, 33, 39, 66, 39, 47, 58, 51], built by repeatedly inserting values into an empty heap. Which element could not have been the last element inserted into this heap?</p>
  </blockquote>
  <p>I know an element is inserted from the leaf, but that leaf element takes position according to its value and this makes me confused.</p>
  <p>I have made the corresponding binary tree:</p>
  <p><img src="https://i.stack.imgur.com/tsncf.jpg" alt="image of heap" /></p>
  <p>How can I determine which values could not have been inserted last?</p>