Create 1175-prime-arrangements.js

Author: everthis

1175-prime-arrangements.js

@@ -0,0 +1,22 @@
+/**
+ * @param {number} n
+ * @return {number}
+ */
+function isPrime(n) {
+  // Corner case
+  if (n <= 1) return false
+  // Check from 2 to n-1
+  for (let i = 2; i < n; i++) if (n % i == 0) return false
+  return true
+}
+
+const numPrimeArrangements = function(n) {
+  let primes = 0 // # of primes.
+  let result = 1
+  const mod = 10 ** 9 + 7
+  for (let i = 2; i <= n; i++) if (isPrime(i)) primes++
+  // Calculate factorials and multiply.
+  for (let i = primes; i >= 1; i--) result = (i * result) % mod
+  for (let i = n - primes; i >= 1; i--) result = (i * result) % mod
+  return result // result of multiplying factorial(primes) with factorial(non-primes)
+}