def to_s
      st = @char
      node = self
      while node != nil
        node = node.up
        break if node==nil
        st = node.char+st
      end
      st
    end
def to_s
      st = @char
      node = self
      while node != nil
        node = node.up
        break if node==nil
        st = node.char+st
      end
      st
    end
Return the complete string from the tree root up to and including this node
 i.e. The total string key for this node from root.