18. 4Sum

2026-03-25 16:58:20 +03:00
parent d99b7d18e3
commit 2464aeff0e
4 changed files with 208 additions and 0 deletions
--- a/4Sum/Solution.cs
+++ b/4Sum/Solution.cs
@@ -0,0 +1,143 @@
+
+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;
+    }
+}