for Codility MissingInteger

MissingInteger.cs

using System;

namespace MissingInteger
{
	class MainClass
	{
		public static void Main (string[] args)
		{
			int[] A = { 1, 3, 5, 4, 1, 2 };
			int[] B = { 1 };
			Solution sol = new Solution ();
			int result = sol.solution (B);
			Console.WriteLine (result);
		}
	}

	public class Solution
	{
		public int solution(int[] input)
		{
			bool[] checkArray = new bool[input.Length + 1];
			int result = checkArray.Length;
			for (int i = 0; i < input.Length; i++) {
				if (input [i] > 0 && input [i] <= input.Length) {
					if (!checkArray [input [i]]) {
						checkArray [input [i]] = true;
					}
				}
			}
			for (int i = 1; i < checkArray.Length; i++) {
				if (!checkArray [i]) {
					result = i;
					break;
				}
			}
			return result;
		}
	}
}