Widest Span

easy

You're stringing festival lights through a tree and want the single longest run of cable that follows branches — between any two nodes, not necessarily through the trunk. Each branch segment (edge) is one meter.

Given the root of a binary tree, return its diameter: the number of edges on the longest path between any two nodes in the tree.

Tests pass a level-order array; implement the core in _diameter on real nodes.

Example 1

in values = [1, 2, 3, 4, 5]

out 3

Longest path: 4 → 2 → 5 won't do (2 edges); 4 → 2 → 1 → 3 has 3 edges.

Example 2

in values = [1, 2]

out 1

Example 3

in values = [1]

out 0

Constraints

· 0 <= number of nodes <= 10^4
· -100 <= node value <= 100
· diameter counts edges, not nodes