c = a;a = b;b = c;
a = a ^ b;b = a ^ b; //a == a ^ b ^ ba = a ^ b; //b == a ^ b ^ a
a = a + b;b = a - b;a = a - b;
b = (a+b) - (a=b);
变量交换(4个方式）
原文：https://www.cnblogs.com/ayeex/p/9515331.html
