def find_node(key)
      return nil if @root == nil
      node = @root.find_vertical(key)
      (node.nil? || node.value.nil? ? nil : node)
    end
def find_node(key)
      return nil if @root == nil
      node = @root.find_vertical(key)
      (node.nil? || node.value.nil? ? nil : node)
    end
Find a node by its key