1175. Prime Arrangements
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Return the number of permutations of 1 to n so that prime numbers are at prime indices (1-indexed.)
(Recall that an integer is prime if and only if it is greater than 1, and cannot be written as a product of two positive integers both smaller than it.)
Since the answer may be large, return the answer modulo 10^9 + 7.
Example 1:
Input: n = 5 Output: 12 Explanation: For example [1,2,5,4,3] is a valid permutation, but [5,2,3,4,1] is not because the prime number 5 is at index 1.
Example 2:
Input: n = 100 Output: 682289015
Constraints:
1 <= n <= 100
Hints
Hint 1
Solve the problem for prime numbers and composite numbers separately.
Hint 2
Multiply the number of permutations of prime numbers over prime indices with the number of permutations of composite numbers over composite indices.
Hint 3
The number of permutations equals the factorial.