Missing integer variation - O(n) solution needed [closed]

Answer 1

C# scored 100%,

Explanation: use of lookup table where we store already seen values from input array, we only care about values that are greater than 0 and lower or equal than length on input array

    public static int solution(int[] A)
    {
        var lookUpArray = new bool[A.Length];

        for (int i = 0; i < A.Length; i++)
            if (A[i] > 0 && A[i] <= A.Length)
                lookUpArray[A[i] - 1] = true;

        for (int i = 0; i < lookUpArray.Length; i++)
            if (!lookUpArray[i])
                return i + 1;

        return A.Length + 1;
    }