Given a Binary Tree, write a function to check whether the given Binary Tree is Complete Binary Tree or not.
A complete binary tree is a binary tree in which every level, except possibly the last, is completely filled, and all nodes are as far left as possible. See the following examples.

The following trees are examples of Complete Binary Trees
   1
 /   \
2     3
 
      1
   /    \
  2       3
 /
4

      1
   /    \
  2      3
 /  \    /
4    5  6

Check whether a given Binary Tree is Complete or not _ Set 1: https://www.geeksforgeeks.org/check-if-a-given-binary-tree-is-complete-tree-or-not/