C语言求助，不用第三变量，如何对调两个数

给：
#include<stdio.h>
main()
{
int a,b;
scanf("%d %d",&a,&b);
printf("Before: a=%d b=%d\n",a,b);
a=a+b;
b=a-b;
a=a-b;
printf("After: a=%d b=%d\n",a,b);
}

a = a + b;
b = a - b;
a = a - b;

a = a + b;
b = a - b;
a = a - b;

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