/// <summary>/// 判斷abcd四個數進行任意四則運算后能不能得出24，每個數字只能用一次/// </summary>/// <param name="a">數字a</param>/// <param name="b">數字b</param>/// <param name="c">數字c</param>/// <param name="d">數字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abcd進行任意四則運算后能不能得出24，每個數字只能用一次/// </summary>/// <param name="a">數字1</param>/// <param name="b">數字2</param>/// <param name="c">數字3</param>/// <param name="d">數字4</param>/// <param name="expression"></param>/// <returns></returns>private static bool TryEach(int a, int b, int c, int d, ref string expression){ expression = ""; //兩個數可以做6種運算：加、減、被減、乘以、除以、除 //四個數共可以進行6*6*6=216種不同次序的四則運算 //初始化數組 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>/// 求兩數進行某一四則運算后的結果/// </summary>/// <param name="x">數字1</param>/// <param name="y">數字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">數字1表達式</param>/// <param name="expressionRight">數字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();}