说明：  4. 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。
(4. To achieve the following elements of the exchange position of the array function: void swap (int a [], int m, int n) the function is able to array a the first m elements with n elements after the exchange of position, that is, the exchange of the former: a1, a2 ,..., aM, aM+1, aM+2 ,..., aM+ N, after the exchange: aM+1, aM+2 ,..., aM+ N, a1, a2, .. ., aM requirements: In addition to an array of a, shall not introduce other array.)