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 the 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:  1 2 3 4 5
Output: 3

(The longest path is 4 → 2 → 1 → 3 or 5 → 2 → 1 → 3, length 3)

Example 2:

Input:  1 2
Output: 1

Constraints:

• The number of nodes in the tree is in the range [1, 10⁴]
• -100 ≤ Node.val ≤ 100

Input format: BFS level-order, space-separated, null for missing nodes.

Output format: Integer — the diameter.