test.cs

using System;

namespace ConsoleApp2 {
    class Program {
        static void Main(string[] args) {
            int p = Rev(1000000003);

            int p2 = FacZeroCount(50);
        }

        static int Rev(int input) {
            int sign = input >= 0 ? 1 : -1;
            int result = 0;
            while (input != 0) {
                try {
                    checked {
                        result = result * 10 + input % 10;
                    }
                } catch {
                    return 0;
                }
                input = (int)Math.Floor(input / 10.0);
            }
            return result * sign;
        }

        static int FacZeroCount(int n) {
            int count = 0;

            int temp = 1;
            for (int i = n; i > 0; i--) {
                temp = temp * i;

                while (temp % 10 > 0) {
                    count++;
                    temp = (int)Math.Floor(temp / 10.0);
                }
            }

            return count;
        }
    }
}