Two Tasks

15. 3Sum/Solution.cs

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+using System;
+using System.Collections.Generic;
+using System.Linq;
+
+namespace _15._3Sum;
+
+public class Solution {
+    
+    class SequenceComparer<T>:IEqualityComparer<IEnumerable<T>>
+    {
+        public bool Equals(IEnumerable<T> seq1,IEnumerable<T> seq2)
+        {
+            return seq1.SequenceEqual(seq2);
+        }
+    
+        public int GetHashCode(IEnumerable<T> seq)
+        {
+            int hash = 1234567;
+            foreach(T elem in seq)
+                hash = unchecked(hash * 37 + elem.GetHashCode());
+            return hash;
+        }
+    }
+    
+    public IList<IList<int>> ThreeSum(int[] nums)
+    {
+        Dictionary<int, int> dict = new Dictionary<int, int>();
+        foreach (var num in nums)
+        {
+            if (dict.ContainsKey(num))
+                dict[num]++;
+            else
+                dict.Add(num, 1);
+        }
+        HashSet<IList<int>> answer = new HashSet<IList<int>>(new SequenceComparer<int>());
+        int[] keys = dict.Keys.ToArray();
+        for (int i = 0; i < keys.Length; i++)
+        {
+            int countI = dict[keys[i]]-1;
+            for (int j = i; j < keys.Length; j++)
+            {
+                int countJ = j == i ? (countI-1) : (dict[keys[j]]-1);
+                int v1 = keys[i];
+                int v2 = keys[j];
+                int v3 = -(v1+v2);
+                if (v3 == v1)
+                    countI--;
+                if (v3 == v2)
+                    countJ--;
+                bool can = dict.ContainsKey(v3);
+                if (can && countI >= 0 && countJ >= 0)
+                {
+                    var list = new List<int>() { v1, v2, v3 };
+                    list.Sort();
+                    answer.Add(list);
+                }
+                countI = dict[keys[i]]-1;
+            }
+        }
+        
+        return answer.ToList();
+    }
+}