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1. 简介

交换两数一般需要借助第三个数当作临时变量；

这里介绍几种不需要临时变量的写法，当然可能还有别的方法。

2. 方法

2.1 借助中间变量

void swap1(int *pa, int *pb)
{int tmp = *pa;*pa     = *pb;*pb = tmp;
}

2.2 利用加减运算符

void swap2(int *pa, int *pb) 
{*pa += *pb;*pb  = *pa - *pb;*pa -= *pb;
}

2.3 利用乘除运算符

void swap3(int *pa, int *pb)
{*pa *= *pb;*pb  = *pa / *pb;*pa /= *pb;
}

2.4 利用异或运算符

void swap4(int *pa, int *pb)
{*pa ^= *pb;*pb ^= *pa;*pa ^= *pb;
}

3. 总结

事实上，我们只需要找到运算符$o{p}_{1}o{p}_2$, 满足
$$\begin{gathered} a=ao{p}_{1}bo{p}_{2}b \\ b=ao{p}_{1}bo{p}_{2}a \end{gathered}$$
就可以不借助中间变量交换两个变量

• 代码

#include <stdio.h>
void swap1(int *pa, int *pb)
{int tmp = *pa;*pa     = *pb;*pb = tmp;
}
void swap2(int *pa, int *pb) 
{*pa += *pb;*pb  = *pa - *pb;*pa -= *pb;
}
void swap3(int *pa, int *pb)
{*pa *= *pb;*pb  = *pa / *pb;*pa /= *pb;
}
void swap4(int *pa, int *pb)
{*pa ^= *pb;*pb ^= *pa;*pa ^= *pb;
}
int main()
{int a0 = 3;int b0 = 4;printf("before swap1: a0: %d, b0: %d\n", a0, b0);swap1( &a0, &b0);printf("after swap1: a0: %d, b0: %d\n", a0, b0);int a1 = 10;int b1 = 22;printf("before swap2: a1: %d, b1: %d\n", a1, b1);swap2( &a1, &b1);printf("after swap2: a1: %d, b1: %d\n", a1, b1);int a2 = -243;int b2 = 829;printf("before swap3: a2: %d, b2: %d\n", a2, b2);swap3( &a2, &b2);printf("after swap3: a2: %d, b2: %d\n", a2, b2);int a3 = 934;int b3 = -234;printf("before swap4: a3: %d, b3: %d\n", a3, b3);swap4( &a3, &b3);printf("after swap4: a3: %d, b3: %d\n", a3, b3);return 0;
}