Let $\Delta_1$ and $\Delta_2$ be two triangulations of the same point set $P_n$. Show that they can be transformed into each other by edge flips. To define an edge flip, let $pqrs$ be vertices (in clockwise order) of a quadrilateral. If $pr$ is an edge in the triangulation, then $pr$ can be flipped into $qs$.
For convex polygon case, it is easy to show there exists a sequence of edge flip that will increase the number of common edges of two different triangulations. But I am stuck in the general case. Any hint?