gistfile1.cs

        static BigInteger ProdTree(int l, int r)
        {
            if (l > r)
                return 1;
            if (l == r)
                return l;
            if (r - l == 1)
                return (BigInteger)l * r;
            int m = (l + r) / 2;
            return ProdTree(l, m) * ProdTree(m + 1, r);
        }

        const int threadCount = 2;

        static Task<BigInteger> AsyncProdTree(int l, int r)
        {
            return Task<BigInteger>.Run(() => ProdTree(l, r));
        }

        static BigInteger FactThreadingTree(int n)
        {
            if (n < 0)
                return 0;
            if (n == 0)
                return 1;
            if (n == 1 || n == 2)
                return n;
            if (n < threadCount+1)
                return ProdTree(2, n);

            Task<BigInteger>[] tasks = new Task<BigInteger>[threadCount];

            tasks[0] = AsyncProdTree(2, n / threadCount);
            for (int i = 1; i < threadCount; i++)
            {
                tasks[i] = AsyncProdTree(((n / threadCount) * i) + 1, (n / threadCount) * (i + 1));
            }

            Task<BigInteger>.WaitAll(tasks);

            BigInteger result = 1;
            for (int i = 0; i < threadCount; i++)
            {
                result *= tasks[i].Result;
            }

            return result;
        }