title: convert a not-full binary search tree to full binary search tree
body: <p>I have a trouble in full binary search tree
  How can I convert a a not-full binary search tree to full binary search tree, such as this example:
  <br>I have a structure of Node in tree:</p>
  
  <p>struct Node
  {
   <br>    int key;
   <br>    Node* left,*right;
  <br>};</p>
  
  <p>and I have a binary tree
  <br>--------18
  <br>------/----\
  <br>-----16---19
  <br>-----/-\----\
  <br>----8--17--20
  <br>---/-----------
  <br>--7------------
  <br>It's not a full binary search tree, it just a binary tree.
  and I want to convert this tree to full binary search tree like:
  <br>---------17
  <br>-------/-----\
  <br>------8------19
  <br>-----/-\-----/---\
  <br>----7-16--18---20
  <br>Please help me to solve it, thank you so much!</p>