/// <summary>
/// 判断abcd四个数进行任意四则运算后能不能得出24，每个数字只能用一次
/// </summary>
/// <param name=\"a\">数字a</param>
/// <param name=\"b\">数字b</param>
/// <param name=\"c\">数字c</param>
/// <param name=\"d\">数字d</param>
/// <param name=\"expression\"></param>
/// <returns></returns>
private static bool Try24(int a, int b, int c, int d, ref string expression)
{
 //a字头
 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字头
 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字头
 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字头
 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>
/// 判断指定顺序的四个数abcd进行任意四则运算后能不能得出24，每个数字只能用一次
/// </summary>
/// <param name=\"a\">数字1</param>
/// <param name=\"b\">数字2</param>
/// <param name=\"c\">数字3</param>
/// <param name=\"d\">数字4</param>
/// <param name=\"expression\"></param>
/// <returns></returns>
private static bool TryEach(int a, int b, int c, int d, ref string expression)
{
 expression = \"\";
 //两个数可以做6种运算：加、减、被减、乘以、除以、除
 //四个数共可以进行6*6*6=216种不同次序的四则运算
 //初始化数组
 for (int i = 0; i < 6 * 6 * 6; i++)
 {
  //a与b间的运算符：i / 36
  //b与c间的运算符：i % 36 / 6
  //c与d间的运算符：i % 6
  //1.运算顺序：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.运算顺序：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.运算顺序：b和c运算，再与a运算，再与d运算
  {
   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.运算顺序：b和c运算，再与d运算，再与a运算
  {
   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.运算顺序：c和d运算，再和b运算，再和a运算
  {
   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>
/// 求两数进行某一四则运算后的结果
/// </summary>
/// <param name=\"x\">数字1</param>
/// <param name=\"y\">数字2</param>
/// <param name=\"method\">（0-5分别代表：加、减、被减、乘以、除以、除）</param>
/// <param name=\"expression\">返回的表达式</param>
/// <param name=\"expressionLeft\">数字1表达式</param>
/// <param name=\"expressionRight\">数字2表达式</param>
/// <returns></returns>
private static int ResultOf(int x, int y, int method,
 ref string expression, string expressionLeft = \"\", string expressionRight = \"\")
{
 //左右表达式之前被判定为无效则不计算，除数为0时不计算
 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:
   {
    //减
    result = x - y;
    expression = string.Format(\"{0}-{1}\",
     expressionLeft == \"\" ? x.ToString() : expressionLeft,
     expressionRight == \"\" ? y.ToString() : expressionRight);
   }
   break;
  case 2:
   {
    //被减
    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;
 }
 //运算不合法，则返回-1，表达式为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();
}