位移操作

1.判断奇偶
  int a = 6;
  a & 1 == 0 可替代 a % 2 == 0

2.交换两值
  int a = 3;
  int b = 5;

  a = a ^ b;
  b = a ^ b;
  a = a ^ b;

  另一种常用的方式
  a = a + b;
  b = a - b;
  a = a - b;

3.正负切换
  int a = 8;
  (~a) + 1 = -8 //取反 + 1