Merge pull request #727 from DarkWarrior703/patch-5

Create double_factorial

math/double_factorial.cpp

@@ -0,0 +1,28 @@
+#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 !! */
+
+uint64_t double_factorial_iterative(uint64_t n) {
+  uint64_t res = 1;
+  for ( uint64_t i = n; i >= 0; i -= 2 ) {
+    if (i == 0 || i == 1) return res;
+    res *= i;
+  }
+}
+
+/* Recursion can be costly for large numbers */
+
+uint64_t double_factorial_recursive(uint64_t n) {
+  if (n <= 1) return 1;
+  return n * double_factorial_recursive(n - 2);
+}
+
+int main() {
+  uint64_t n{};
+  std::cin >> n;
+  assert(n >= 0);
+  std::cout << double_factorial_iterative(n);
+}