PermMissingElem

        /// <summary>
        /// Solution A
        /// 100/100
        /// </summary>
        /// <param name="A"></param>
        /// <returns></returns>
        public static int solution(int[] A)
        {
            HashSet<int> hs = new HashSet<int>();

            foreach (int i in A)
            {
                hs.Add(i);
            }

            for (int i = 1; i <= A.Length + 1; i++)
            {
                if (!hs.Contains(i))
                    return i;
            }
            return 0;
        }

        /// <summary>
        /// Solution B
        /// 100/100
        /// </summary>
        /// <param name="A"></param>
        /// <returns></returns>
        public static int solution(int[] A)
        {

            //int N = A.Length;
            long N = A.Length;

            //总和可能超出int最大值，所以用long
            long sumFull = (N + 1) * (N + 2) / 2;

            //long sumLack = A.Sum();
            long sumLack = 0;

            foreach (int i in A)
                sumLack += i;

            return (int)(sumFull - sumLack);
        }

        /// <summary>
        /// Solution C
        /// 100/100
        /// </summary>
        /// <param name="A"></param>
        /// <returns></returns>
        public static int solution(int[] A)
        {
            Array.Sort(A);
            int N = A.Length;
            for (int i = 1; i <= N; i++)
            {
                if (A[i - 1] != i)
                    return i;
            }
            return N + 1;
        }