在计算机辅助设计(CAD)中，有一个经典问题：消除隐藏线（被其它图形遮住的线段）。你需要设计一个软件，帮助建筑师绘制城市的侧视轮廓图。为了方便处理，限定所有的建筑物都是矩形的，而且全部建立在同一水平面上。每个建筑物用一个三元组表示（Li,Hi,Ri）其中Li和Ri分别是建筑物i的左右边缘坐标，Hi是建筑物i的高度。

-In computer-aided design (CAD), there is a classic problem: the elimination of hidden lines (covered by other graphics of the segment). You need to design a software architect to help draw the outline of the city side of the map. In order to facilitate the processing, all the buildings are limited to rectangular, and all built on the same horizontal surface. Each group of buildings that use a ternary (Li, Hi, Ri) in which Li and Ri are the left and right edge of the building i coordinates, Hi is the height of the building i.