/// <summary>
/// 判斷abcd四個數(shù)進行任意四則運算后能不能得出24，每個數(shù)字只能用一次
/// </summary>
/// <param name="a">數(shù)字a</param>
/// <param name="b">數(shù)字b</param>
/// <param name="c">數(shù)字c</param>
/// <param name="d">數(shù)字d</param>
/// <param name="expression"></param>
/// <returns></returns>
private static bool Try24(int a, int b, int c, int d, ref string expression)
{
 //a字頭
 if (TryEach(a, b, c, d, ref expression)) return true;
 if (TryEach(a, b, d, c, ref expression)) return true;
 if (TryEach(a, c, b, d, ref expression)) return true;
 if (TryEach(a, c, d, b, ref expression)) return true;
 if (TryEach(a, d, b, c, ref expression)) return true;
 if (TryEach(a, d, c, b, ref expression)) return true;
 //b字頭
 if (TryEach(b, a, c, d, ref expression)) return true;
 if (TryEach(b, a, d, c, ref expression)) return true;
 if (TryEach(b, c, a, d, ref expression)) return true;
 if (TryEach(b, c, d, a, ref expression)) return true;
 if (TryEach(b, d, a, c, ref expression)) return true;
 if (TryEach(b, d, c, a, ref expression)) return true;
 //c字頭
 if (TryEach(c, a, b, d, ref expression)) return true;
 if (TryEach(c, a, d, b, ref expression)) return true;
 if (TryEach(c, b, a, c, ref expression)) return true;
 if (TryEach(c, b, c, a, ref expression)) return true;
 if (TryEach(c, d, a, b, ref expression)) return true;
 if (TryEach(c, d, b, a, ref expression)) return true;
 //d字頭
 if (TryEach(d, a, b, c, ref expression)) return true;
 if (TryEach(d, a, c, b, ref expression)) return true;
 if (TryEach(d, b, a, c, ref expression)) return true;
 if (TryEach(d, b, c, a, ref expression)) return true;
 if (TryEach(d, c, a, b, ref expression)) return true;
 if (TryEach(d, c, b, a, ref expression)) return true;
 return false;
}
/// <summary>
/// 判斷指定順序的四個數(shù)abcd進行任意四則運算后能不能得出24，每個數(shù)字只能用一次
/// </summary>
/// <param name="a">數(shù)字1</param>
/// <param name="b">數(shù)字2</param>
/// <param name="c">數(shù)字3</param>
/// <param name="d">數(shù)字4</param>
/// <param name="expression"></param>
/// <returns></returns>
private static bool TryEach(int a, int b, int c, int d, ref string expression)
{
 expression = "";
 //兩個數(shù)可以做6種運算：加、減、被減、乘以、除以、除
 //四個數(shù)共可以進行6*6*6=216種不同次序的四則運算
 //初始化數(shù)組
 for (int i = 0; i < 6 * 6 * 6; i++)
 {
  //a與b間的運算符：i / 36
  //b與c間的運算符：i % 36 / 6
  //c與d間的運算符：i % 6
  //1.運算順序：a和b，再和c，再和d
  {
   string expression1 = "", expression2 = "";
   int temp1 = ResultOf(a, b, i / 36, ref expression1);
   int temp2 = ResultOf(temp1, c, i % 36 / 6, ref expression2, expression1);
   int result = ResultOf(temp2, d, i % 6, ref expression, expression2);
   if (result == 24) return true;
  }
  //2.運算順序：a和b，c和d，前面部分和后面部分
  {
   string expression1 = "", expression2 = "";
   int temp1 = ResultOf(a, b, i / 36, ref expression1);
   int temp2 = ResultOf(c, d, i % 6, ref expression2);
   int result = ResultOf(temp1, temp2, i % 36 / 6,
    ref expression, expression1, expression2);
   if (result == 24) return true;
  }
  //3.運算順序：b和c運算，再與a運算，再與d運算
  {
   string expression1 = "", expression2 = "";
   int temp1 = ResultOf(b, c, i % 36 / 6, ref expression1);
   int temp2 = ResultOf(a, temp1, i / 36, ref expression2, "", expression1);
   int result = ResultOf(temp2, d, i % 6, ref expression, expression2);
   if (result == 24) return true;
  }
  //4.運算順序：b和c運算，再與d運算，再與a運算
  {
   string expression1 = "", expression2 = "";
   int temp1 = ResultOf(b, c, i % 36 / 6, ref expression1);
   int temp2 = ResultOf(temp1, d, i % 6, ref expression2, expression1);
   int result = ResultOf(a, temp2, i / 36, ref expression, "", expression2);
   if (result == 24) return true;
  }
  //5.運算順序：c和d運算，再和b運算，再和a運算
  {
   string expression1 = "", expression2 = "";
   int temp1 = ResultOf(c, d, i % 6, ref expression1);
   int temp2 = ResultOf(b, temp1, i % 36 / 6, ref expression2, "", expression1);
   int result = ResultOf(a, temp2, i / 36, ref expression, "", expression2);
   if (result == 24) return true;
  }
 }
 expression = "Abandoned";
 return false;
}
/// <summary>
/// 求兩數(shù)進行某一四則運算后的結果
/// </summary>
/// <param name="x">數(shù)字1</param>
/// <param name="y">數(shù)字2</param>
/// <param name="method">（0-5分別代表：加、減、被減、乘以、除以、除）</param>
/// <param name="expression">返回的表達式</param>
/// <param name="expressionLeft">數(shù)字1表達式</param>
/// <param name="expressionRight">數(shù)字2表達式</param>
/// <returns></returns>
private static int ResultOf(int x, int y, int method,
 ref string expression, string expressionLeft = "", string expressionRight = "")
{
 //左右表達式之前被判定為無效則不計算，除數(shù)為0時不計算
 if (expressionLeft == "Abandoned" || expressionRight == "Abandoned" ||
  (x == 0 && method == 5) || (y == 0 && method == 4))
 {
  expression = "Abandoned";
  return -1;
 }
 int result = 0;
 switch (method)
 {
  case 0:
   {
    //加
    result = x + y;
    expression = string.Format("{0}+{1}",
     expressionLeft == "" ? x.ToString() : expressionLeft,
     expressionRight == "" ? y.ToString() : expressionRight);
   }
   break;
  case 1:
   {
    //減
    result = x - y;
    expression = string.Format("{0}-{1}",
     expressionLeft == "" ? x.ToString() : expressionLeft,
     expressionRight == "" ? y.ToString() : expressionRight);
   }
   break;
  case 2:
   {
    //被減
    result = y - x;
    expression = string.Format("{1}-{0}",
     expressionLeft == "" ? x.ToString() : expressionLeft,
     expressionRight == "" ? y.ToString() : expressionRight);
   }
   break;
  case 3:
   {
    //乘以
    result = x * y;
    expression = string.Format("({0})*({1})",
     expressionLeft == "" ? x.ToString() : expressionLeft,
     expressionRight == "" ? y.ToString() : expressionRight);
   }
   break;
  case 4:
   {
    //除以
    if (x % y == 0)
    {
     result = x / y;
     expression = string.Format("({0})/({1})",
      expressionLeft == "" ? x.ToString() : expressionLeft,
      expressionRight == "" ? y.ToString() : expressionRight);
    }
    else
    {
     expression = "Abandoned";
    }
   }
   break;
  case 5:
   {
    //除
    if (y % x == 0)
    {
     result = y / x;
     expression = string.Format("({1})/({0})",
      expressionLeft == "" ? x.ToString() : expressionLeft,
      expressionRight == "" ? y.ToString() : expressionRight);
    }
    else
    {
     expression = "Abandoned";
    }
   }
   break;
 }
 //運算不合法，則返回-1，表達式為Abandoned，
 if (expression == "Abandoned")
 {
  return -1;
 }
 return result;
}

static void Main(string[] args)
{
 string s = "";
 bool try24;
 try24 = Try24(1, 2, 3, 4, ref s); Console.WriteLine("1,2,3,4: " + s);
 try24 = Try24(1, 3, 5, 7, ref s); Console.WriteLine("1,3,5,7: " + s);
 try24 = Try24(2, 3, 6, 4, ref s); Console.WriteLine("2,3,6,4: " + s);
 try24 = Try24(5, 7, 9, 4, ref s); Console.WriteLine("5,7,9,4: " + s);
 try24 = Try24(6, 3, 4, 5, ref s); Console.WriteLine("6,3,4,5: " + s);
 try24 = Try24(9, 5, 2, 7, ref s); Console.WriteLine("9,5,2,7: " + s);
 try24 = Try24(4, 9, 8, 3, ref s); Console.WriteLine("4,9,8,3: " + s);
 try24 = Try24(4, 9, 8, 2, ref s); Console.WriteLine("4,9,8,2: " + s);
 try24 = Try24(10, 10, 10, 10, ref s); Console.WriteLine("10,10,10,10: " + s);
 Console.ReadLine();
}