create 0168-excel-sheet-column-title.cs

Author: NotADucc

csharp/0168-excel-sheet-column-title.cs

@@ -0,0 +1,22 @@
+public class Solution {
+
+    private const int _ASCII_BASE = 64;
+
+    public string ConvertToTitle(int columnNumber)
+    {
+        StringBuilder sb = new StringBuilder();
+
+        while (columnNumber > 0)
+        {
+            int rest = columnNumber % 26;
+            rest = rest == 0 ? 26 : rest;
+            
+            sb.Insert(0, (char)(rest + _ASCII_BASE));
+
+            columnNumber -= rest;
+            columnNumber /= 26;
+        }
+
+        return sb.ToString();
+    }
+}