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 <= 100struct Solution;
impl Solution {
fn number_of_primes(n: usize) -> i32 {
let mut a: Vec<bool> = vec![true; n + 1];
a[0] = false;
a[1] = false;
let mut i: usize = 2;
while i * i <= n {
if a[i] {
let mut j: usize = 2;
while i * j <= n {
a[i * j] = false;
j += 1;
}
}
i += 1;
}
let mut res = 0;
for k in 0..=n {
if a[k] {
res += 1;
}
}
res
}
fn num_prime_arrangements(n: i32) -> i32 {
let primes = Self::number_of_primes(n as usize);
let mut product = 1i64;
for i in 1..=primes {
product *= i as i64;
product %= 1_000_000_007;
}
for i in 1..=(n - primes) {
product *= i as i64;
product %= 1_000_000_007;
}
product as i32
}
}
#[test]
fn test() {
assert_eq!(Solution::num_prime_arrangements(5), 12);
assert_eq!(Solution::num_prime_arrangements(100), 682_289_015);
}