剑指Offer[51] 数组中的逆序对

Related Topics:
“树状数组”: https://leetcode.com/tag/binary-indexed-tree/
“线段树”: https://leetcode.com/tag/segment-tree/
“数组”: https://leetcode.com/tag/array/
“二分查找”: https://leetcode.com/tag/binary-search/
“分治”: https://leetcode.com/tag/divide-and-conquer/
“有序集合”: https://leetcode.com/tag/ordered-set/
“归并排序”: https://leetcode.com/tag/merge-sort/

Problem:

在数组中的两个数字，如果前面一个数字大于后面的数字，则这两个数字组成一个逆序对。输入一个数组，求出这个数组中的逆序对的总数。

示例 1:

输入: [7,5,6,4]
输出: 5

限制：

`0 <= 数组长度 <= 50000