1.利用中间变量 temp
int a ，b，temp；   
temp = a;
a = b;
b = a;

2.利用算术运算
int a ,b;
a = b - a;
b = b -a;
a = b+a;

3.利用异或运算
int a , b ;
a = a^b;
b = a^b;
a = a^b;

a = b + (b = a) * 0;
DotNet专用，是利用堆栈的先进后出的性质实现的