From a77f0d3afe5c737fe083abe9cd8cd6b1e3986fe3 Mon Sep 17 00:00:00 2001
From: DarkWarrior703 <56077342+DarkWarrior703@users.noreply.github.com>
Date: Fri, 24 Apr 2020 21:01:14 +0300
Subject: [PATCH] Create double_factorial

Calculates the double factorial of an integer
---
 math/double_factorial | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)
 create mode 100644 math/double_factorial

diff --git a/math/double_factorial b/math/double_factorial
new file mode 100644
index 000000000..5f41337f2
--- /dev/null
+++ b/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);
+}