笔者预祝大伙圣诞节快乐，不要滥砍滥伐啊。
牛逼的小伙伴们有新招，3D打印一棵tree，不浪费。但是这个3D没有大伙想象的那么简单，
正在3D打印中的圣诞树。
如果有一个平底，在平底上一层层叠加成型，那么就好办了，可是它是椎体。
这是3D打印的圣诞树，椎体3D打印可没那么容易哈。
研究人员正在展示成果。如果真的打印一颗与圣诞树一样高度和大小的3D打印圣诞树，可以吗？
图片和资料来源: Simon Fraser University
欲详细了解这种3D打印技术，请参考如下文献：
标题：Approximate Pyramidal Shape Decomposition
摘要：A shape is pyramidal if it has a flat base with the remaining boundary forming a height function over the base. Pyramidal shapes are optimal for molding, casting, and layered 3D printing. However, many common objects are not pyramidal. We introduce an algorithm for approximate pyramidal shape decomposition. The general exact pyramidal decomposition problem is NP-hard. We turn this problem into an NP-complete problem which admits a practical solution. Specifically, we link pyramidal decomposition to the Exact Cover Problem (ECP). Given an input shape S, we develop clustering schemes to derive a set of building blocks for approximate pyramidal parts of S. The building blocks are then combined to yield a set of candidate pyramidal parts. Finally, we employ Knuth’s Algorithm X over the candidate parts to obtain solutions to ECP as pyramidal shape decompositions. Our solution is equally applicable to 2D or 3D shapes, and to shapes with polygonal or smooth boundaries, with or without holes. We demonstrate our algorithm on numerous shapes and evaluate its performance.