using static System.Console;

using System.Text;

​

public class Program {

    public static void Main() {

        WriteLine("Digite uma frase: ");

        var frase = ReadLine();

        WriteLine($"Original string: {frase}");

        var invertida = new StringBuilder(frase.Length);

        foreach (var chr in frase) invertida.Append(char.ToLower(chr == ' ' ? '0' : chr));

        WriteLine($"Character array: {invertida}");

    }

}