AtCoder Grand Contest 023
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Problem Statement
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Snuke has an integer sequence $A$ whose length is $N$. He likes permutations of $(1, 2, ..., N)$, $P$, that satisfy the following condition:
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$P_i \leq A_i$for all$i$($1 \leq i \leq N$).
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Snuke is interested in the inversion numbers of such permutations. Find the sum of the inversion numbers over all permutations that satisfy the condition. Since this can be extremely large, compute the sum modulo $10^9+7$.
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Note
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The inversion number of a sequence $Z$ whose length $N$ is the number of pairs of integers $i$ and $j$ ( $1 \leq i < j \leq N$ ) such that $Z_i > Z_j$.
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Constraints
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$1 \leq N \leq 2 \times 10^5$$1 \leq A_i \leq N$($1 \leq i \leq N$)- All values in input are integers.
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Input
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Input is given from Standard Input in the following format:
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N
A_1 A_2 ... A_N
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Output
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Print the sum of the inversion numbers over all permutations that satisfy the condition.
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样例
Input #1
3
2 3 3
Output #1
4
There are four permutations that satisfy the condition: $(1,2,3)$, $(1,3,2)$, $(2,1,3)$ and $(2,3,1)$. The inversion numbers of these permutations are $0$, $1$, $1$ and $2$, respectively, for a total of $4$.
Input #2
6
4 2 5 1 6 3
Output #2
7
Only one permutation $(4,2,5,1,6,3)$ satisfies the condition. The inversion number of this permutation is $7$, so the answer is $7$.
Input #3
5
4 4 4 4 4
Output #3
0
No permutation satisfies the condition.
Input #4
30
22 30 15 20 10 29 11 29 28 11 26 10 18 28 22 5 29 16 24 24 27 10 21 30 29 19 28 27 18 23
Output #4
848414012
题目信息
题面与样例版权归 AtCoder 及出题人所有;中文翻译由 AI 生成(CI 管线),如有出入请以原文为准。