title: Find whether the following tree exists in the list of million of binary search trees
body: <p>For example,</p>
  
  <p>consider the following tree's, check whether they exist in the list of BST.</p>
  
  <pre><code>           5
            / \
           4   6
          / \
         1   3
  
  
         3
        / \
       2   4
  </code></pre>
  
  <p>How to approach to this problem?</p>