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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.
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).
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