one.rb

require_relative 'jubilib/binary_tree'

def validate_and_return_minmax(node)
  left_valid, left_min, left_max = node.left.nil? ?
    [true, node.val, node.val] :
    validate_and_return_minmax(node.left)
  return [false, 0, 0] unless left_valid
  right_valid, right_min, right_max = node.right.nil? ?
    [true, node.val, node.val] :
    validate_and_return_minmax(node.right)
  return [false, 0, 0] unless right_valid

  valid = ((node.left.nil? or left_max < node.val) and (node.right.nil? or right_min > node.val))
  return [false, 0, 0] unless valid
  return [true, left_min, right_max]
end

def is_valid_bst(root)
  return true if root.nil?
  valid, min, max = validate_and_return_minmax(root)
  return valid
end