Given a Binary Tree, write a function to check whether the given Binary Tree is a Complete Binary Tree or not.
A full binary tree is defined as a binary tree in which all nodes have either zero or two child nodes. Conversely, there is no node in a full binary tree, which has one child node.
The following tree is a complete binary tree
10
/ \
20 30
/ \
40 50
18
/ \
15 30
The following tree is not a complete binary tree
1
/ \
2 3
/ \
5 6