This problem was asked by Microsoft.
Let X be a set of n intervals on the real line. We say that a set of points P "stabs" X if every interval in X contains at least one point in P. Compute the smallest set of points that stabs X.
For example, given the intervals [(1, 4), (4, 5), (7, 9), (9, 12)], you should return [4, 9].