不借助临时变量，进行两个整数的交换

举例：输入 a = 6, b = 2 输出 a = 2, b =6

    var a = 6;
    var b = 2;
    b = a - b; //b=4
    a = a - b // a=2
    b = a + b //b=6
    console.log(a, b)