1.4.14 #

public static class FourSum
{
    /// <summary>
    /// 输出数组中所有和为 0 的四元组。
    /// </summary>
    /// <param name="a">包含所有元素的数组。</param>
    public static void PrintAll(long[] a)
    {
        var n = a.Length;
        for (var i = 0; i < n; i++)
        {
            for (var j = i + 1; j < n; j++)
            {
                for (var k = j + 1; k < n; k++)
                {
                    for (var l = k + 1; l < n; l++)
                    {
                        if (a[i] + a[j] + a[k] + a[l] == 0)
                        {
                            Console.WriteLine($"{a[i]} + {a[j]} + {a[k]} + {a[l]} = 0");
                        }
                    }
                }
            }
        }
    }

    /// <summary>
    /// 计算和为零的四元组的数量。
    /// </summary>
    /// <param name="a">包含所有元素的数组。</param>
    /// <returns>和为零的四元组的数量。</returns>
    public static int Count(long[] a)
    {
        var n = a.Length;
        var cnt = 0;

        for (var i = 0; i < n; i++)
        {
            for (var j = i + 1; j < n; j++)
            {
                for (var k = j + 1; k < n; k++)
                {
                    for (var l = k + 1; l < n; l++)
                    {
                        if (a[i] + a[j] + a[k] + a[l] == 0)
                        {
                            cnt++;
                        }
                    }
                }
            }
        }

        return cnt;
    }
}