https://www.jianshu.com/p/91f2c503e62f

It could reduce the tree size very much

leetcode 699. Falling Squares

练习了两道题，感觉基本掌握了segment tree 308. Range Sum Query 2D - Mutable 307. Range Sum Query - Mutable

但是感觉segment写起来很笨重，一大堆的操作和helper函数，看到很多BIT的写法更加的精简，值得仔细研究一下