说明：  实现下面的数组元素交换位置函数：void swap(int a[], int m, int n) 该函数能够把数组a的前m个元素与后n个元素交换位置，即， 交换前：a1,a2,...,aM, aM+1,aM+2,...,aM+N 交换后：aM+1,aM+2,...,aM+N, a1,a2,...,aM （要求：除数组a外，不得引入其它数组。）
( implement the following array element exchange position function: void swap (int a [], int m, int n) This function can be put n elements of the array a before and after the m-th element exchange position, that is, before the exchange: a1, a2, ..., aM, aM+1, aM+2, ..., after aM+N exchange: aM+1, aM+2, ..., aM+N, a1, a2, ..., aM (requirements: In addition to the array a, shall not introduce other array.))