Given a binary tree, you need to compute the length of the diameter of the tree. The diameter of a binary tree is the length of the longest path between any two nodes in a tree. This path may or may not pass through the root.
Given a binary tree
Return 3, which is the length of the path [4,2,1,3] or [5,2,1,3].
The length of path between two nodes is represented by the number of edges between them.