说明：  线段树是在计算几何领域非常重要的一种数据结构，常在各大ACM比赛中现身，本文由浅入深，用往年经典的ACM题目来介绍这个强大数据结构
(Segment tree is in the field of computational geometry is very important as a data structure, often in major competitions ACM show, this article easy-to-digest, ACM with a classic topic in previous years to introduce this powerful data structure)