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PERMUT1 - Permutations |
Let A = [a1, a2, ... an] be a permutation of integers 1, 2, ... n. A pair of indices (i, j), 1 <= i <= j <= n, is an inversion of the permutation A if ai > aj. We are given integers n > 0 and k >= 0. What is the number of n-element permutations containing exactly k inversions?
For instance, the number of 4-element permutations with exactly 1 inversion equals 3.
Task
Write a program which for each data set from a sequence of several data sets:
- reads integers n and k from input,
- computes the number of n-element permutations with exactly k inversions,
- writes the result to output.
Input
The first line of the input file contains one integer d, 1 <= d <= 10, which is the number of data sets. The data sets follow. Each data set occupies one line of the input file and contains two integers n (1 <= n <= 12) and k (0 <= k <= 98) separated by a single space.
Output
The i-th line of the output file should contain one integer - the number of n-element permutations with exactly k inversions.
Example
Input: 1 4 1 Output: 3
Added by: | adrian |
Date: | 2004-06-22 |
Time limit: | 3s |
Source limit: | 50000B |
Memory limit: | 1536MB |
Cluster: | Cube (Intel G860) |
Languages: | All |
Resource: | III Polish Collegiate Team Programming Contest (AMPPZ), 1998 |
hide comments
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2020-07-07 03:08:11
https://stackoverflow.com/questions/19372991/number-of-n-element-permutations-with-exactly-k-inversions |
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2020-05-05 03:26:45
A good question, can be solved using DP with bitmasking |
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2020-04-27 22:53:49
The input constraints are wrong. They test cases in which n > 12. I was able to solve the problem by assuming n <= 20. |
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2020-04-14 13:28:54
Very good question. I loved solving it. |
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2020-04-05 04:35:52
nice question for beginners. |
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2020-03-22 20:52:20
@taiken for n=7 , k=2 , answer is 20 , n = 7 , k=3 , answer is 49, n = 8 , k =3 , answer is 76. |
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2018-12-20 16:00:30
just ensure dp[1][0]==1 .. got 2 WA because of it.. |
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2018-10-26 20:18:35
one of the best problem i ever had on dp... |
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2018-06-28 15:32:34
good problem:) nice and elegant dp solution |
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2018-05-30 20:26:29
func(n,k)=summation func(n-1,k-i)...........and then AC:) |