other node relationships two nodes that are children of the same parent are siblings node is external if has no children node is internal if it has one or more children external nodes are also known as leaves example in section we discussed the hierarchical relationship between files and directories in computer' file systemalthough at the time we did not emphasize the nomenclature of file system as tree in figure we revisit an earlier example we see that the internal nodes of the tree are associated with directories and the leaves 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 "/user/rt/coursescs cs grades homeworkshw hw hw programspr pr projectspr papersbuylow sellhigh grades demosmarket figure tree representing portion of file system node is an ancestor of node if or is an ancestor of the parent of converselywe say that node is descendant of node if is an ancestor of for examplein figure cs is an ancestor of papers/and pr is descendant of cs the subtree of rooted at node is the tree consisting of all the descendants of in (including itselfin figure the subtree rooted at cs consists of the nodes cs /gradeshomeworks/programs/hw hw hw pr pr and pr edges and paths in trees an edge of tree is pair of nodes (uvsuch that is the parent of vor vice versa path of is sequence of nodes such that any two consecutive nodes in the sequence form an edge for examplethe tree in figure contains the path (cs /projects/demos/market