快速查找数组中重复值

public static void chaozaoxiantong()
{
    int[] a = new int[] { 12, 23, 18, 15, 12 };
    ArrayList al = new ArrayList();
    // int n=0;
    for (int i = 0; i < a.Length - 1; i++)
    {
        for (int p = 0; p < a.Length - 1 - i; p++)
        {
            if (a[i] != a[i + 1])
            {
                int num = a[i];
                if (!al.Contains(num))
                {
                    al.Add(a[i]);
                }
                // al[i]=a[i];
            }
        }
    }
    for (int y = 0; y < al.Count; y++)
    {
        Console.WriteLine("重复的数据:{0}", al[y]);
    }
    Console.WriteLine("数组大小:{0}", a.Length);
}