归并排序求逆序数 3402
https://blog.csdn.net/qq_39897867/article/details/79211809
https://blog.csdn.net/maverickfw/article/details/53115740
https://blog.csdn.net/loi_yzs/article/category/6487429
https://blog.csdn.net/Mashiro_ylb/article/details/78376439?utm_source=blogxgwz3
https://blog.csdn.net/ourfutr2330/article/details/53216442
https://blog.csdn.net/qq_37067419/article/details/78616506
https://blog.csdn.net/mashiro_ylb/article/details/78469151
https://www.luogu.org/problemnew/show/P3957
https://blog.csdn.net/qq_37067419/article/details/78616506
find函数
https://www.cnblogs.com/wkfvawl/p/9429128.html