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Count number of arrays with k-inversions
By aakarshmadhavan, history, 6 years ago, In English

I'm trying to come up with a recursive solution for this problem but I haven't been able to so far. Any advice/pointers here?

I tried 0-1 knapsack style but there is an issue with backtracking the array.

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6 years ago, # |
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It appears that the statement is actually asking for the number of permutations of 1... n with k inversions. If In(t) is the number of permutations of size n with t inversions, then . This recurrence can be calculated in O(nt) using prefix sums on In.

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