Diameter of a Binary Tree

Problem

Given the root of a binary tree, return the length of the diameter — the longest path between any two nodes (path need not pass through the root).

Example

    1
   / \
  2   3
 / \
4   5
Output: 3  -- path 4→2→1→3 or 4→2→5

Constraints

• 1 ≤ nodes ≤ 10^4
• Node values are unique

Follow-up

Extend your solution to return the actual path nodes, not just the length.