树中元素及元素之间的关系 A tree is a recursive abstract data type which is a collection of zero or more nodes each containing an element of type elemtype with the following hierarchi- cal structure: either (a)The empty structure(with no nodes) or (b)A node N containing element E with a finite number (zero or more)of associated disjoint tree structures (that is,they have no common nodes).These trees are known as subtrees of N.N is called the root of the tree. 你如何理解这两个概念：层次结构、递归ADT?树中元素及元素之间的关系 你如何理解这两个概念：层次结构、递归ADT？