|
An internal node of a tree is any node which has degree greater than one. Or, phrased in rooted tree terminology, the internal nodes of a tree are the nodes which have at least one child node.
$$\xymatrix{ & {\color{red}\bullet} \ar@{-}[dl] \ar@{-}[dr] & & & \\ \bullet & & {\color{red}\bullet} \ar@{-}[dr]\ar@{-}[dl] & & \\ & {\color{red}\bullet} \ar@{-}[dl] & & \bullet & \\ \bullet & & & & }$$
Figure: A tree with internal nodes highlighted in red.
| 
