Hi Codeforces community,
Recently I have come across a problem which turned out to be tough for me. I hope that I can get some help from you.
Problem Statement
A permutation A of first N integers from 1 to N is good if it has exactly K good positions. A position i is good only if abs(A[i] - i) = 1. The task is to count how many permutation of first N integers like that, modulo 109 + 7.
Input
N and K, 1 ≤ N ≤ 1000, 0 ≤ K ≤ N
Output
Number of permutation of first N integers from 1 to N that has exactly K good positions, modulo 109 + 7
Example
For N = 3, K = 2, there are 4 permutations that has 2 good positions. They are (1, 3, 2) , (2, 1, 3) , (2, 3, 1) , (3, 1, 2).
You may want to submit your solution here (written in Vietnamese, required SPOJ account): http://www.spoj.com/PTIT/problems/P172PROI/
I think it is a DP problem although I could not come up with a solution or any idea. Any help will be appreciated.
Thanks in advance.
