if has no children node is internal if it has one or more children external nodes are also known as leaves example in most operating systemsfiles are organized hierarchically into nested directories (also called folders)which are presented to the user in the form of tree (see figure more specificallythe internal nodes of the tree are associated with directories and the external nodes are associated with regular files in the unix and linux operating systemsthe root of the tree is appropriately called the "root directory,and is represented by the symbol "figure system tree representing portion of file node is an ancestor of node if or is an ancestor of the parent of converselywe say that node is descendent of node if is an ancestor of for examplein figure cs is an ancestor of papers/and pr is descendent of cs the subtree of rooted at node is the tree consisting of all the descendents of in (including itselfin figure the subtree rooted at cs consists of the nodes cs /gradeshomeworks/programs/hw hw hw pr pr and pr