#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cerr.tie(0);
  int n, tmp;
  vector<pair<int, int> > a;
  cin >> n;
  for (int i = 0; i < n; i++) cin >> tmp, a.push_back(make_pair(tmp, i + 1));
  sort(a.begin(), a.end());
  int c1 = 0, c2 = 0, s1 = 0, s2 = 0;
  vector<int> t1, t2;
  int l = 0, r = n - 1;
  while (l <= r) {
    if (s1 > s2) {
      if (c2 - c1 <= 0) {
        c2++;
        s2 += a[r].first;
        t2.push_back(a[r].second);
        r--;
      } else {
        c1++;
        s1 += a[l].first;
        t1.push_back(a[l].second);
        l++;
      }
    } else {
      if (c2 - c1 <= 0) {
        c2++;
        s2 += a[l].first;
        t2.push_back(a[l].second);
        l++;
      } else {
        c1++;
        s1 += a[r].first;
        t1.push_back(a[r].second);
        r--;
      }
    }
  }
  cout << c1 << endl;
  for (int i = 0; i < c1; i++) cout << t1[i] << " ";
  cout << endl;
  cout << c2 << endl;
  for (int i = 0; i < c2; i++) cout << t2[i] << " ";
  return 0;
}
