Given the root of a binary tree, return 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.
The length of a path between two nodes is represented by the number of edges between them.
[1, 10^4]-100 <= Node.val <= 100root = [1,2,3,4,5]3root = [1,2]1