LeetCode in Kotlin
1175. Prime Arrangements
Easy
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
Solution
@Suppress("NAME_SHADOWING")
class Solution {
fun numPrimeArrangements(n: Int): Int {
var n = n
val a = intArrayOf(
2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83,
89, 97
)
var c = 0
while (c < 25 && n >= a[c]) {
c++
}
val m = 1000000007
var res = 1L
while (n - c > 0) {
res *= (n - c).toLong()
res %= m.toLong()
n--
}
while (c > 0) {
res *= c.toLong()
res %= m.toLong()
c--
}
return res.toInt()
}
}