LeetCode-in-All

124. Binary Tree Maximum Path Sum

Hard

A path in a binary tree is a sequence of nodes where each pair of adjacent nodes in the sequence has an edge connecting them. A node can only appear in the sequence at most once. Note that the path does not need to pass through the root.

The path sum of a path is the sum of the node’s values in the path.

Given the root of a binary tree, return the maximum path sum of any non-empty path.

Example 1:

Input: root = [1,2,3]

Output: 6

Explanation: The optimal path is 2 -> 1 -> 3 with a path sum of 2 + 1 + 3 = 6.

Example 2:

Input: root = [-10,9,20,null,null,15,7]

Output: 42

Explanation: The optimal path is 15 -> 20 -> 7 with a path sum of 15 + 20 + 7 = 42.

Constraints:

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 max_path_sum(root :: TreeNode.t | nil) :: integer
  def max_path_sum(%TreeNode{val: val, left: nil, right: nil}), do: val
  def max_path_sum(root) do
    max_path(root, -1001) |> elem(1)
  end

  def max_path(%TreeNode{val: val, left: left, right: right}, max) do
    {lval, lmax} = max_path(left, max)
    {rval, rmax} = max_path(right, max)

    {
      [(lval + val), (rval + val), val] |> Enum.max(), 
      [max, val, val + lval, val + rval, val + lval + rval, lmax, rmax] |> Enum.max()
    }
  end
  def max_path(nil, max), do: {0, max}
end