Exercises and problems: try to solve the skyline problem

Suppose you are given a series of rectangular boxes laid along the positive x axis. Each box is defined by 3 integer numbers; a left x value, a height along the y axis, and a right x value. For example one triple might be (1 11 5), meaning that the box extends horizontally from x=1 to x=5, with a height on the y axis of 11 units. The rectangular boxes may overlap or intersect as well as stand alone. For example, the three boxes represented by (1 11 5), (2 6 7) and (3 13 9) represent two intersecting boxes, (1 11 5) (3 13 9) and one completely subsumed one (2 6 7).