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CSharp · Console

/Program.cs

using System;
using System.Linq;
					
public class Program
{
	public static void Main()
	{
		Console.WriteLine(!IsValid(""));
		Console.WriteLine(!IsValid("010"));
		Console.WriteLine(!IsValid("001"));
		Console.WriteLine(!IsValid("12a"));
		Console.WriteLine(!IsValid("123"));
		Console.WriteLine(!IsValid("1234"));
		Console.WriteLine(IsValid("12"));
	}
	
	static bool IsValid(string input)
	{
		return !string.IsNullOrEmpty(input) && input.Length < 3 && !input.StartsWith("0") && input.All(char.IsDigit); 
	}
}