< mathematics , data > A directed acyclic graph ; i.e. a graph wherein there is only one route between any pair of nodes , and there is a notion of "toward top of the tree" (i.e. the root node ), and its opposite direction, toward the leaves . A tree with n nodes has n-1 edges.
Although maybe not part of the widest definition of a tree, a common constraint is that no node can have more than one parent. Moreover, for some applications, it is necessary to consider a node's daughter nodes to be an ordered list , instead of merely a set .
As a data structure in computer programs, trees are used in everything from B-trees in databases and file systems , to game trees in game theory , to syntax trees in a human or computer languages .
(1998-11-12)