using System;

using System.Linq;

public class Program

{

    static int[]f(int[]c){int j=0;int h()=>c.Contains(++j)?h():j;return c.Select((e,i)=>Array.IndexOf(c,e)<i?h():e).ToArray();}

    public static void Main()

    {

        test(new int[] { });

        test(new int[] { 5 });

        test(new int[] { 1, 4, 2, 5, 3, 6 });

        test(new int[] { 3, 3, 3, 3, 3, 3 });

        test(new int[] { 6, 6, 4, 4, 2, 2 });

        test(new int[] { 2147483647, 2, 2147483647, 2 });

    }

    static void test(int[] input)

    {

        var output = f(input);

        Console.WriteLine("Input:  " + string.Join(", ", input));

        Console.WriteLine("Output: " + string.Join(", ", output));

        Console.WriteLine();

    }

}