A tree is a finite nonempty set of elements , it is an abstract model of hierarchical structure. Application:Organization chartsFile systemsProgramming environments 名词解释：Root：根node without parentSiblings：兄弟节点 nodes share the same parentInternal node ：内部节点node with at least one childExternal node（leaf）：叶节点node without childrenAncestors of a node :祖先节点Descendant of a node ：后裔 术语：
Depth of a node: number of ancestors
Height of a tree: maximum depth of any node
Degree of a node: the number of its children
Degree of a tree: the maximum number of its node.
Subtree: tree consisting of a node and its descendants
