4.

4.#

 1#include <iostream>
 2
 3using namespace std;
 4
 5void fun(int a[], int i, int j) {
 6  int t = a[i];
 7  a[i]  = a[j];
 8  a[j]  = t;
 9  if (i < j) {
10    fun(a, i + 1, j - 1);
11  }
12}
13
14int main() {
15  int a[] = {1, 2, 3, 4, 5};
16  fun(a, 0, 4);
17  for (int i = 0; i < 5; i++) {
18    cout << a[i];
19  }
20  return 0;
21}