为什么这个代码是冰川

我本来希望这段代码执行得很快，但是对于一个有5000个元素的列表，它在我杀死它之前运行了超过20分钟。

是因为list.contains()吗?

任何想法?

     public static int PNum(int N)
     {
        List<int> result = new List<int>();
        for (int i = 1; i <= N; i++)
            result.Add(i * (3 * i - 1) / 2);
        int len = result.Count;
        int minDiff = 10000;
        int sum = 0, diff = 0;
        for (int i = 0; i <= len - 1; i++)
        {
            for (int j = i + 1; j <= len - 1; j++)
            {
                diff = result[j] - result[i];
                if (result.Contains(diff))
                {
                    sum = result[i] + result[j];
                    if (result.Contains(sum))
                    {
                        if (diff < minDiff)
                            diff = minDiff;
                    }
                }
            }
        }
        return minDiff;
    }