[#0166] feat(leetcode): add README, unit test and C# solution

problems/0166-fraction-to-recurring-decimal/csharp/Program.cs

@@ -5,20 +5,64 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Text;
 
-public class Solution {
-    // TODO: 根據題意調整簽名
-    public int Solve(int[] nums) {
-        // 實作解法
-        return 0;
+public class Solution
+{
+    public string FractionToDecimal(int numerator, int denominator)
+    {
+        if (numerator == 0) return "0";
+
+        long num = numerator;
+        long den = denominator;
+
+        StringBuilder result = new StringBuilder();
+        bool isNegative = (num < 0) ^ (den < 0);
+        if (isNegative) result.Append('-');
+
+        num = Math.Abs(num);
+        den = Math.Abs(den);
+
+        long integerPart = num / den;
+        result.Append(integerPart);
+
+        long remainder = num % den;
+        if (remainder == 0) return result.ToString();
+
+        result.Append('.');
+        result.Append(CalculateDecimalPart(remainder, den));
+
+        return result.ToString();
+    }
+
+    private string CalculateDecimalPart(long remainder, long denominator)
+    {
+        StringBuilder decimalBuilder = new StringBuilder();
+        Dictionary<long, int> remainderMap = new Dictionary<long, int>();
+
+        while (remainder != 0)
+        {
+            if (remainderMap.TryGetValue(remainder, out int repeatIndex))
+            {
+                decimalBuilder.Insert(repeatIndex, '(');
+                decimalBuilder.Append(')');
+                break;
+            }
+
+            remainderMap[remainder] = decimalBuilder.Length;
+            remainder *= 10;
+            long digit = remainder / denominator;
+            decimalBuilder.Append(digit);
+            remainder %= denominator;
+        }
+
+        return decimalBuilder.ToString();
     }
 }
 
 public class Program {
     public static void Main() {
         var s = new Solution();
-        // TODO: 可加入簡單輸入/輸出測試
-        Console.WriteLine("Hello LeetCode 166!");
+        
     }
 }
-