LeetCode-in-Java

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:

Solution

public class Solution {
    public int numPrimeArrangements(int n) {
        int[] a = {
            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
        };
        int c = 0;
        while (c < 25 && n >= a[c]) {
            c++;
        }
        int m = 1000000007;
        long res = 1L;
        while ((n - c) > 0) {
            res *= (n - c);
            res %= m;
            n--;
        }
        while (c > 0) {
            res *= c;
            res %= m;
            c--;
        }
        return (int) res;
    }
}