MissingInteger

using System;
using System.Collections.Generic;
using System.Linq;

class Solution {
    public int solution(int[] A) {
        // write your code in C# 6.0 with .NET 4.5 (Mono)
        var set = new HashSet<int>(A);
        foreach (int a in A){
            if (a > 0) {
                set.Add(a);
            }
        }
        // var array2 = set.ToArray();
        // Display the resulting array.
        // Console.WriteLine(string.Join(",", array2));
        for (int i = 1; i <= A.Length + 1; i++) {
            if (!set.Contains(i)) {
                //Console.WriteLine(set.Contains(i));
                return i;
            }
        }
        return 0;
    }
}