using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

​

namespace ConsoleApp4

{

    class Program

    {

        static void Main(string[] args)

        {

            /*

             * Input: [1,2,3,[1,[2,3]]]

             * Output: [1,2,3,1,2,3]

            */

            List<object> input = new List<object> {

                1,

                2,

                3,

                new object[]{

                    1,

                    new object[]{

                        2,

                        3

                    }

                }

            };

​

            List<object> output = FlattenArray(input);

​

            foreach (var item in output)

            {

                Console.WriteLine(item.ToString());

            }

            Console.ReadLine();

        }

​

        private static List<object> FlattenArray(IEnumerable<object> input)

        {

            return input.Select(x => x is object[] array ? FlattenArray(array) : x).ToList();

        }

    }

}

​