TheAlgorithms-C-Plus-Plus/math/double_factorial.cpp

/**
 * @file
 * @brief Compute double factorial: \f$n!!\f$
 *
 * 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.
 * <br/>It is also called as semifactorial of a number and is denoted by
 * \f$n!!\f$
 */

#include <cassert>
#include <iostream>

/** Compute double factorial using iterative method
 */
uint64_t double_factorial_iterative(uint64_t n) {
    uint64_t res = 1;
    for (uint64_t i = n;; i -= 2) {
        if (i == 0 || i == 1)
            return res;
        res *= i;
    }
    return res;
}

/** Compute double factorial using resursive method.
 * <br/>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);
}

/// main function
int main() {
    uint64_t n;
    std::cin >> n;
    assert(n >= 0);
    std::cout << double_factorial_iterative(n);
}