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.

Example 1:

    Input: root = [1,2,3,4,5]
    Output: 3
    Explanation: 3 is the length of the path [4,2,1,3] or [5,2,1,3].

Example 2:

    Input: root = [1,2]
    Output: 1

Constraints:

- The number of nodes in the tree is in the range `[1, 104]`.
- `-100 <= Node.val <= 100`