The 1-Steiner tree problem is a variant of the Steiner tree problem, where we are only allowed to use a single additional point. In this thesis we present an efficient approach of Georgakopoulos and Papadimitrou to solving the 1-Steiner tree problem.
The original Steiner tree problem is known to be NP-hard. On the other hand it is fairly easy to show that the 1-Steiner tree problem is polynomial. With the use of Dirichlet diagrams, their overlay, and an efficient updating of minimal spanning trees with preprocessing we show, that the 1-Steiner tree of a set of $n$ terminals can be computed in $O(n^2)$ time.
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