树是由n(n≥0)个结点组成的有限集合T。n=0的树称为空树；对n>0的树，有:(1)仅有一个特殊的结点称为根结点，根结点没有前驱结点；(2)当n>1时，除根结点外其余的结点分为m(m>0)个互不相交的有限集合T1,T2，…，Tm，其中每个集合Ti本身又是一棵结构和树类似的子树。

-Tree is n (n ≥ 0) consisting of nodes finite set T. n = 0 of the tree known as the empty tree　for n>　0 of the tree, there is: (1) has only one special node called the root node, root node there is no precursor node　(2) when n>　1 when, with the exception of root node except the node is divided into m (m>　0) a finite set of mutually exclusive cross-T1, T2, ..., Tm, one of each set Ti is itself a structure and are similar subtree tree .