FactorialArray.cs

using System;

namespace FactorialArray
{
    class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());
            int[] num = new int[n];
            int digit = 1;   //位數   
            num[0] = 1;
            for (int i = 1; i <= n; i++)
            {
                //所有位數都要乘
                for (int j = 0; j < n; j++)
                    num[j] *= i;
                for (int k = 0; k < n - 1; k++)
                {
                    //超過個位數字時要進位
                    if (num[k] > 9)
                    {
                        num[k + 1] = num[k + 1] + num[k] / 10;
                        num[k] = num[k] % 10;
                    }
                }
            }

            //查看有幾位數
            for (int i = n - 1; i >= 0; i--)
            {
                if (num[i] != 0)
                {
                    digit = i + 1;
                    break;
                }
            }

            //印出結果
            Console.Write(n + "!=");            
            for (int i = digit - 1; i >= 0; i--)
                Console.Write(num[i]);
        }
    }
}