重新看C语言时，一个小发现，在交换两个变量的值时，一般设个变量：

           int a  = 13, b = 27;

           int  c;

           c = a;

           a = b;

           b = c;

还可以不用中间变量：

           a = a + b;

           b = a - b;

           a = a - b;

    或：

          a = a ^ b;

          b = b ^ a;

                                                                        a = a ^ b;                                                          

                                                                                         //  ^为位异或运算符（异或运算符是直接对位进行运算的，所以运行结果是最快的）