"interview practice" solution for problem: write a function that validates a binary tree

bst_validate

require 'test/unit'

class Node
   attr_accessor :val, :left, :right
   def initialize val 
      @val = val 
   end 

   def >(other)
      @val > other.val
   end 

   def <(other)
      @val < other.val
   end 

end

def validate_tree root
   if root.nil?
      return true
   elsif root.left.nil?
      return true if root.right.nil? 
      return false if root > root.right
   elsif root.right.nil?
      return false if root < root.left
   end 

   return false if root.left > root.right ||
                   root.left > root ||
                   root > root.right
   return false unless validate_tree(root.left)
   return false unless validate_tree(root.right)
   true
end

class BinaryTreeValidatorTest < Test::Unit::TestCase
   def test_base_case
      root = Node.new 7
      root.left = Node.new 8
      root.right = Node.new 9
      assert !validate_tree(root), "Tree is invalid but validate returns true"
   end 
   def test_fails_for_invalid_trees
      root = Node.new 7
      root.left = Node.new 4
      root.right = Node.new 8
      root.left.left = Node.new 5
      assert !validate_tree(root), "Tree is invalid and not complete, but validate returns true"
   end 

   def test_passes_for_valid_trees
      root = Node.new 5
      root.left = Node.new 4
      root.right = Node.new 6
      assert validate_tree(root), "Valid tree returns invalid"
   end 
end