#include <bits/stdc++.h>
using namespace std;
int a[100005], b[100005];
bool compare(int c, int d) { return a[c - 1] < a[d - 1]; }
int main() {
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    b[i] = i + 1;
  }
  sort(b, b + n, compare);
  cout << (n + 1) / 2 << endl;
  for (int i = 0; i < n; i += 2) cout << b[i] << ' ';
  cout << endl << n / 2 << endl;
  for (int i = 1; i < n; i += 2) cout << b[i] << ' ';
  cout << endl;
  return 0;
}
