Create double_factorial

Calculates the double factorial of an integer

math/double_factorial

@@ -0,0 +1,30 @@
+#include <iostream>
+#include <cassert>
+
+/* Double factorial of a non-negative integer n,
+is defined as the product of all the integers from 1 to n
+that have the same parity (odd or even) as n.
+It is also called as semifactorial of a number and is denoted by !! */
+
+unsigned long long double_factorial_iterative(unsigned int n){
+  unsigned long long res = 1;
+  for(unsigned long long i = n; i >= 0; i -= 2){
+    if(i == 0 || i == 1) return res;
+    res *= i;
+  }
+}
+
+/* Recursion can be costly for large numbers */
+
+unsigned long long double_factorial_recursive(unsigned int n){
+  if(n <= 1) return 1;
+  return n * double_factorial_recursive(n - 2);
+}
+
+int main()
+{
+  int n{};
+  std::cin >> n;
+  assert(n >= 0);
+  std::cout << double_factorial_iterative(n);
+}