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第一种方法：
a = a + b;
b = a – b;
a = a – b;
第二种方法：
a=a^b;
b=a^b;
a=a^b;
或者
a^=b^=a^=b;
原理是：相同的两数相异为0，任何数与0相异保留不变
但不推荐这么使用，c FAQs里有说明，用异或准确来讲是undefined，Not portbably
Q: Here’s a slick expression:
a ^= b ^= a ^= b
It swaps a and b without using a temporary.
A: Not portably, it doesn’t. It attempts to modify the variable a twice between sequence points, so its behavior is undefined.
For example, it has been reported that when given the code
int a = 123, b = 7654;
a ^= b ^= a ^= b;
the SCO Optimizing C compiler (icc) sets b to 123 and a to 0.