title: Double Factorial
signature: doubleFactorial(n) {
test_code: import 'dart:convert';
  
  // SOLUTION CODE
  // ============================================
  PLACEHOLDER_CODE_BODY
  
  // TESTING CODE 
  // ============================================
  bool validateSolution(int actual, int expected){
      return actual.toString() == expected.toString();
  }
  
  String driver(n, int expected){
      try
      {
          if (validateSolution(PLACEHOLDER_FN_NAME(n), expected)){
              return "PASSED";
          }
          return "FAILED";
      }catch(exception_object){
          return exception_object.runtimeType.toString();
      }
  }
  
  int main() {
  
      print("TEST-0...${driver(
          15, 
          2027025)
      }");
  
      print("TEST-1...${driver(
          21, 
          13749310575)
      }");
  
      return 0;
  }
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 );
  }