Difficulty: Medium
Tags: LeetCode, Medium, Tree, Depth-First Search, Breadth-First Search, Graph, Topological Sort, leetcode-1245, O(|V| + |E|), O(|E|)

Problem

The diameter of a tree is the number of edges in the longest path in that tree.

There is an undirected tree of n nodes labeled from 0 to n - 1. You are given a 2D array edges where edges.length == n - 1 and edges[i] = [a_i, b_i] indicates that there is an undirected edge between nodes a_i and b_i in the tree.

Return the diameter of the tree.

Example 1:

Input: edges = [[0,1],[0,2]]
Output: 2
Explanation: The longest path of the tree is the path 1 - 0 - 2.

Example 2:

Input: edges = [[0,1],[1,2],[2,3],[1,4],[4,5]]
Output: 4
Explanation: The longest path of the tree is the path 3 - 2 - 1 - 4 - 5.

Constraints:

n == edges.length + 1
1 <= n <= 10⁴
0 <= a_i, b_i < n
a_i != b_i