CF396D.On Sum of Number of Inversions in Permutations
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题目描述
You are given a permutation p . Calculate the total number of inversions in all permutations that lexicographically do not exceed the given one.
As this number can be very large, print it modulo 1000000007 (109+7) .
输入格式
The first line contains a single integer n ( 1<=n<=106 ) — the length of the permutation. The second line contains n distinct integers p1,p2,...,pn ( 1<=pi<=n ).
输出格式
Print a single number — the answer to the problem modulo 1000000007 (109+7) .
输入输出样例
输入#1
2 2 1
输出#1
1
输入#2
3 2 1 3
输出#2
2
说明/提示
Permutation p of length n is the sequence that consists of n distinct integers, each of them is from 1 to n .
An inversion of permutation p1,p2,...,pn is a pair of indexes (i,j) , such that i<j and p_{i}>p_{j} .
Permutation a do not exceed permutation b lexicographically, if either a=b or there exists such number i , for which the following logical condition fulfills: AND (a_{i}<b_{i}) .