27 Segment Tree or BIT
开刷四年了，终于下定决心学起了BIT和segment tree。
简单介绍：
Binary Index Tree （Fenwick Tree）
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L439 Segment Tree Build II
L248 Count of Smaller Number
L247 Segment Tree Query II
207 Interval Sum II
206 Interval Sum
L205 Interval Minimum Number
L203 Segment Tree Modify
L202 Segment Tree Query
L201 Segment Tree Build
--------------下面的可以用线段树或者Binary Index Tree-----------
L249 Count of Smaller Number Before itself
315 Count of Smaller Numbers After Self
308 Range Sum Query 2D - Mutable
493 Reverse Pairs -- 可以用merge sort添
1649 Create Sorted Array through Instructions -- 也可以merge sort
other related：
303 Range Sum Query - immutable -- prefixSum
304 Range Sum Query 2D - immutable -- prefix Sum
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