s0543_diameter_of_binary_tree

543. Diameter of Binary Tree

Easy

Given the root of a binary tree, return the length of the diameter of the tree.

The diameter of a binary tree is the length of the longest path between any two nodes in a tree. This path may or may not pass through the root.

The length of a path between two nodes is represented by the number of edges between them.

Example 1:

Input: root = [1,2,3,4,5]

Output: 3

Explanation: 3 is the length of the path [4,2,1,3] or [5,2,1,3].

Example 2:

Input: root = [1,2]

Output: 1

Constraints:

• The number of nodes in the tree is in the range [1, 104].
• -100 <= Node.val <= 100

Solution

# Definition for a binary tree node.
#
# defmodule TreeNode do
#   @type t :: %__MODULE__{
#           val: integer,
#           left: TreeNode.t() | nil,
#           right: TreeNode.t() | nil
#         }
#   defstruct val: 0, left: nil, right: nil
# end

defmodule Solution do
  @spec diameter_of_binary_tree(root :: TreeNode.t | nil) :: integer
  def diameter_of_binary_tree(root),
    do: rec_diameter(root)
        |> then(fn {max, _} -> max end)

  @spec rec_diameter(root :: TreeNode.t | nil) :: {integer, integer}
  defp rec_diameter(nil), do: {0, 0}
  defp rec_diameter(%TreeNode{left: l, right: r}) do
    {l_max, l_longest} = rec_diameter(l)
    {r_max, r_longest} = rec_diameter(r)
    {max(max(l_max, r_max), l_longest + r_longest), max(l_longest, r_longest) + 1}
  end
end