Day i do not know

46. Permutations/46. Permutations.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net6.0</TargetFramework>
+    <RootNamespace>_46._Permutations</RootNamespace>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

46. Permutations/Program.cs

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+namespace _46._Permutations;
+
+internal class Program
+{
+    static void Main(string[] args)
+    {
+        var arr = new int[] { 1,2,3 };
+        var l = new Solution().Permute(arr);
+        foreach (var row in l)
+            Console.WriteLine(String.Join(" ", row));
+    }
+}

46. Permutations/Solution.cs

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+namespace _46._Permutations;
+
+public class Solution
+{
+    public IList<IList<int>> Permute(int[] nums)
+    {
+        List<IList<int>> l = new List<IList<int>>();
+        Recursion(l, new List<int>(), nums.ToHashSet());
+        return l;
+    }
+
+    private void Recursion(List<IList<int>> result, List<int> cur, HashSet<int> aviable)
+    {
+        if(aviable.Count == 0)
+        {
+            result.Add(cur);
+            return;
+        }
+
+        foreach(var av in aviable)
+        {
+            var newAviable = aviable.ToHashSet();
+            newAviable.Remove(av);
+
+            var newCur = cur.ToList();
+            newCur.Add(av);
+            Recursion(result, newCur, newAviable);
+        }
+    }
+}