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using System;
using System.Linq;
 
public class Test
{
    private static int MakePalindrome(int left, int? cen)
    {
		string right = new string(left.ToString().Reverse().ToArray());
		string cenStr = cen.HasValue ? cen.ToString() : string.Empty;
		return int.Parse(left.ToString() + cenStr + right);
	}
 
	private static int MaxPalindrome(int maxValue)
	{
		string digits = maxValue.ToString();
		if (digits.Length == 1) return maxValue;
		bool hasCen = digits.Length % 2 > 0;
		int left = int.Parse(digits.Substring(0, digits.Length / 2));
		int oLeft = left;
		int? cen = hasCen ? int.Parse(digits.Substring(digits.Length / 2, 1)) : (int?) null;
		int result = MakePalindrome(left, cen);
		if (result > maxValue)
		{
			if (hasCen)
			{
				cen--;
				if (cen < 0)
				{
					cen = 9;
					left--;
				}
			}
			else
			{
				left--;
			}
 
			if (left == 0 || left.ToString().Length < oLeft.ToString().Length)
			{
				result = int.Parse(new string('9', digits.Length - 1));
			}
			else
			{
				result = MakePalindrome(left, cen);
			}
		}
		return result;
	}
 
	public static void Main()
	{
        int lastPalindrome = 0;
        for (int i = 5000; i < 10000; i++)
        {
            int newPalindrome = MaxPalindrome(i);
            if (lastPalindrome == 0) lastPalindrome = newPalindrome;
            Console.WriteLine("{0}/{1}", i, newPalindrome);
            if (lastPalindrome != newPalindrome)
            {
                if (i != newPalindrome) {
                    Console.WriteLine("***{0}/{1}", i, newPalindrome); 
                    throw new InvalidProgramException("Incorrect palindrome");
                }
                lastPalindrome = newPalindrome;
            }
        }
	}
}

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Success #stdin #stdout 0.1s 34032KB

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Standard input is empty

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https://ideone.com/StCw4u

language:

C# (gmcs 5.20.1)

created:

visibility:

public

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