title: Double Factorial
signature: doubleFactorial :: Integer -> Integer
  doubleFactorial n = 
test_code: import Text.Printf
  import Control.Exception (catch, SomeException)
  import Data.Typeable
  import qualified Data.List as List
  import qualified Data.Map as Map
  import qualified Data.Set as Set
  
  -- SOLUTION CODE
  -- ============================================
  
  PLACEHOLDER_CODE_BODY
  
  -- TESTING CODE 
  -- ============================================
  validateSolution :: Integer -> Integer -> Bool
  validateSolution actual expected = do
      actual == expected
  
  
  driver :: Integer -> Integer -> String
  driver n expected = do
      if (validateSolution (PLACEHOLDER_FN_NAME n) expected)
          then "PASSED"
      else "FAILED"
  
  main = do
      printf "TEST-0...%s\n" (driver (15) (2027025) )
  
      printf "TEST-1...%s\n" (driver (21) (13749310575) )
  
source_py: def double_factorial(n):
      if ((n == 0) or (n == 1)):
          return 1
      return (n * double_factorial((n - 2)))
source_cpp: unsigned int double_factorial(unsigned int n) {
    if ( n == 0 || n == 1 ) return 1;
    return n * double_factorial ( n - 2 );
  }