using System;public static class B3639 private static void Main string s=ConsoleReadLineSplit ; forint j=sizeofssizeofstring-1;j=0;j-- ifcharIsDigitsj0 forint i=sjLength-1

The code has a few errors. Here's the corrected version:

using System;

public static class B3639
{
    private static void Main()
    {
        string[] s = Console.ReadLine().Split(' ');
        for (int j = s.Length - 1; j >= 0; j--)
        {
            if (char.IsDigit(s[j][0]))
            {
                for (int i = s[j].Length - 1; i >= 0; i--)
                {
                    Console.Write(s[j][i]);
                }
            }
            else
            {
                for (int i = 0; i < s[j].Length; i++)
                {
                    if (char.IsLower(s[j][i]))
                    {
                        Console.Write(char.ToUpper(s[j][i]));
                    }
                    else if (char.IsUpper(s[j][i]))
                    {
                        Console.Write(char.ToLower(s[j][i]));
                    }
                }
            }
            Console.Write(' ');
        }
        Console.Write('\n');
    }
}

This code should compile and run without any errors