Codeforces Round #298 (Div. 2)--D. Handshakes

#include <stdio.h>
#include <algorithm>
#include <set>

using namespace std;

#define forn(i, n) for (int i = 0; i < int(n); i++)

typedef pair<int, int> pii;
const int N = 1000001;

int n;
set<pii> a[3];
int ans[N];

int main() {
    scanf("%d", &n);
    forn(i, n) {
        int x;
        scanf("%d", &x);
        a[x % 3].insert(make_pair(x, i));
    }

    int cur = 0;
    forn(i, n) {
        set <pii> :: iterator it = a[cur % 3].lower_bound(make_pair(cur, -1));  
        pii c;
        if (it != a[cur % 3].end() && (*it).first == cur) {
            c = *(it);
            ans[i] = c.second + 1;
        } else {
            if (it == a[cur % 3].begin()) {
                puts("Impossible");
                return 0;
            } else {
                it--;
                c = *(it);
                ans[i] = c.second + 1;
            }
        }       

        a[cur % 3].erase(c);
        cur = c.first + 1;
    }

    puts("Possible");
    forn(i, n)
        printf("%d ", ans[i]);

    return 0;
}