利用异或实现两个数交换原理

异或基本性质：

• 相同为 0， 不同为 1
• x ^ x = 0
• x ^ 0 = x

a, b交换

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

证：

(1) a = a ^ b   =>  a = a ^ b, b = b
(2) b = a ^ b  =>  a = a ^ b, b = a ^ b ^ b = a ^ 0 = a
(3) a = a ^ b  =>  a = a ^ b ^ a = b ^ 0 = b

备注：需考虑内存溢出