Day i do not know

784. Letter Case Permutation/784. Letter Case Permutation.csproj

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+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>net6.0</TargetFramework>
+    <RootNamespace>_784._Letter_Case_Permutation</RootNamespace>
+    <ImplicitUsings>enable</ImplicitUsings>
+    <Nullable>enable</Nullable>
+  </PropertyGroup>
+
+</Project>

784. Letter Case Permutation/Program.cs

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+namespace _784._Letter_Case_Permutation;
+
+internal class Program
+{
+    static void Main(string[] args)
+    {
+        Console.WriteLine(String.Join(" ", new Solution().LetterCasePermutation("hello")));
+    }
+}

784. Letter Case Permutation/Solution.cs

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+using System.Text;
+
+namespace _784._Letter_Case_Permutation;
+
+public class Solution
+{
+    public IList<string> LetterCasePermutation(string s)
+    {
+        HashSet<string> set = new HashSet<string>();
+        StringBuilder sb = new StringBuilder(s);
+        for (int i = 0; i < Math.Pow(2, s.Length); i++)
+        {
+            UppercasePerMask(sb, i);
+            set.Add(sb.ToString());
+        }
+        return set.ToList();
+    }
+
+    private void UppercasePerMask(StringBuilder sb, int mask)
+    {
+        for (int i = sb.Length - 1; i >= 0; i--)
+        {
+            if (char.IsLetter(sb[i]))
+                sb[i] = mask % 2 == 1 ? char.ToUpper(sb[i]) : char.ToLower(sb[i]);
+            mask >>= 1;
+        }
+    }
+}