Day i do not know

77. Combinations/77. Combinations.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net6.0</TargetFramework>
+    <RootNamespace>_77._Combinations</RootNamespace>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

77. Combinations/Program.cs

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+namespace _77._Combinations;
+
+internal class Program
+{
+    static void Main(string[] args)
+    {
+        var res = new Solution().Combine(4, 2);
+        foreach(var row in res)
+            Console.WriteLine(string.Join(" ", row));
+    }
+}

77. Combinations/Solution.cs

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+namespace _77._Combinations;
+
+public class Solution
+{
+    public IList<IList<int>> Combine(int n, int k)
+    {
+        List<IList<int>> res = new List<IList<int>>();
+        Recursion(res, new List<int>(), 1, n, k);
+        return res;
+    }
+
+    private void Recursion(List<IList<int>> result, List<int> cur, int minNum, int maxNum, int count)
+    {
+        if (cur.Count == count)
+        {
+            result.Add(cur.ToList());
+            return;
+        }
+        for (int toAdd = minNum; toAdd <= maxNum; toAdd++)
+        {
+            cur.Add(toAdd);
+            Recursion(result, cur, toAdd + 1, maxNum, count);
+            cur.RemoveAt(cur.Count-1);
+        }
+    }
+}