Given an undirected tree with $n$ nodes labeled from $0$ to $n - 1$, represented by a 2D array edges where $edges.length == n - 1$. Each $edges[i] = [a_i, b_i]$ denotes an undirected edge between nodes $a_i$ and $b_i$. Your task is to return the diameter of the tree.

The diameter of a tree is the number of ...