How should I practice Count Array Derangements?

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Problem · Dynamic Programming

● Medium

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A derangement is a permutation in which no element remains in its original position.

You are given an integer n. Start with the array [1, 2, ..., n] in ascending order. Return the number of distinct derangements of this array.

Because the answer can be large, return it modulo 10^9 + 7.

findDerangement(n: int) → int

n = 3return = 2

The two derangements are [2, 3, 1] and [3, 1, 2].

n = 2return = 1

The only derangement is [2, 1].