code: def tree_has_single_path(self, node):
          
          num_children = len(node.children)
          if num_children > 1:
              return False
          elif num_children == 0:
              return True
          else:
              return True and self.tree_has_single_path(node.children[0])
docs: If there is a single path in the tree,
          return True, else return False.