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An undirected tree (below right) can always be directed by picking one of the nodes as the root and orienting all edges away from it. Picking node A as the root in the undirected graph below yields the directed tree on the left.

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Nodes that have degree one are called leaf nodes (C,G,E,H,I above). The other nodes are called internal (or interior) node (A,B,D,F above).