Given a binary tree, find out its diameter.
Diameter of a tree is the distance of the longest path between two nodes.
Example, the tree given below the diameter of the tree is 2.
We can observe that the diameter of a tree is maximum of the following:
1. Diameter of the right subtree
2. Diameter of the lef tsubtree.
3. 1 + (height of left subtree) + (height of right subtree)