max_tree_dist.rb

# Definition for a binary tree node.
# class TreeNode
#     attr_accessor :val, :left, :right
#     def initialize(val)
#         @val = val
#         @left, @right = nil, nil
#     end
# end

# @param {TreeNode} root
# @return {Integer}
def diameter_of_binary_tree(root)
  return 0 unless root

  graph = [Node.new(root.val)]
  build_graph(root, graph[0], graph)
  ans = 0
  graph.each do |g_node|
    visited = Set.new
    $max = 0
    dfs(g_node, visited)
    ans = [ans, $max].max
  end
  
  ans
end

def dfs(node, visited, dst = 0)
  visited << node
  
  node.adj.each do |neighbour|
    next if visited.include?(neighbour)
    
    visited << neighbour
    dfs(neighbour, visited, dst + 1)
  end
  
  $max = [$max, dst].max
end

def build_graph(tree_node, g_node, graph)
  if tree_node.left
    left = Node.new(tree_node.left.val)
    g_node.adj << left
    left.adj << g_node
    graph << left
    build_graph(tree_node.left, left, graph)
  end
  if tree_node.right
    right = Node.new(tree_node.right.val)
    g_node.adj << right
    right.adj << g_node
    graph << right
    build_graph(tree_node.right, right, graph)
  end
  
  g_node
end

class Node
  attr_reader :val
  attr_accessor :adj
  
  def initialize(val)
    @val = val
    @adj = []
  end
end