public class Solution
{
    record struct TwoSumSource(int A, int B)
    {
        public override string ToString()
        {
            return $"({A}+{B}={A + B})";
        }
    }

    record struct AnswerRow(int? N0 = null, int? N1 = null, int? N2 = null, int? N3 = null)
    {

        public void PushNum(int num)
        {
            if (N0 == null)
            {
                N0 = num;
            }
            else if (N1 == null)
            {
                N1 = num;
                if (N0 > N1)
                {
                    (N0, N1) = (N1, N0);
                }
            }
            else if (N2 == null)
            {
                N2 = num;
                if (N1 > N2)
                {
                    (N1, N2) = (N2, N1);
                }
                if (N0 > N1)
                {
                    (N0, N1) = (N1, N0);
                }
            }
            else if (N3 == null)
            {
                N3 = num;
                if (N2 > N3)
                {
                    (N2, N3) = (N3, N2);
                }
                if (N1 > N2)
                {
                    (N1, N2) = (N2, N1);
                }
                if (N0 > N1)
                {
                    (N0, N1) = (N1, N0);
                }
            }
            else if (num < N3)
            {
                N3 = num;
                if (N2 > N3)
                {
                    (N2, N3) = (N3, N2);
                }
                if (N1 > N2)
                {
                    (N1, N2) = (N2, N1);
                }
                if (N0 > N1)
                {
                    (N0, N1) = (N1, N0);
                }
            }
        }
    }


    public IList<IList<int>> FourSum(int[] nums, int target)
    {
        var dict = new Dictionary<int, int>();
        // суммы из двух слагаемых, все возможные
        var twoSums = new Dictionary<long, HashSet<TwoSumSource>>();
        for (var i = 0; i < nums.Length; i++)
        {
            var num1 = nums[i];
            dict.TryAdd(num1, 0);
            dict[num1]++;
            for (var j = i + 1; j < nums.Length; j++)
            {
                var num2 = nums[j];
                var sum = num1 + num2;
                twoSums.TryAdd(sum, new HashSet<TwoSumSource>());
                twoSums[sum].Add(new TwoSumSource() { A = num1, B = num2 });
            }
        }

        var rows = new HashSet<AnswerRow>();
        // словарь для требуемоего количества чисел
        var needDict = new Dictionary<int, int>();
        foreach (var kp1 in dict)
        {
            var needThreeSum = target - kp1.Key;
            needDict.TryAdd(kp1.Key, 0);
            needDict[kp1.Key]++;
            foreach (var kp2 in dict)
            {
                needDict.TryAdd(kp2.Key, 0);
                needDict[kp2.Key]++;

                var needTwoSum = (long)needThreeSum - kp2.Key;
                if (twoSums.TryGetValue(needTwoSum, out var curTwoSums))
                {
                    foreach (var curTwoSum in curTwoSums)
                    {
                        needDict.TryAdd(curTwoSum.A, 0);
                        needDict[curTwoSum.A]++;

                        needDict.TryAdd(curTwoSum.B, 0);
                        needDict[curTwoSum.B]++;

                        if (needDict.All(kp => dict[kp.Key] >= kp.Value))
                        {
                            var row = new AnswerRow();
                            row.PushNum(kp1.Key);
                            row.PushNum(kp2.Key);
                            row.PushNum(curTwoSum.A);
                            row.PushNum(curTwoSum.B);
                            rows.Add(row);
                        }

                        needDict[curTwoSum.A]--;
                        needDict[curTwoSum.B]--;
                    }
                }
                needDict[kp2.Key]--;
            }
            needDict.Clear();
        }
        var answer = new List<IList<int>>(rows.Count);
        foreach (var row in rows)
            answer.Add(new List<int> { row.N0!.Value, row.N1!.Value, row.N2!.Value, row.N3!.Value });
        return answer;
    }
}