Codeforces-534D Handshakes

传送门

N个数，第i个数代表i之前的还未参加比赛的人的人数。每来够三个人可以参加比赛，她们可以随时加入比赛（也可以暂时不加入），求一个可能的到达顺序满足题意

#include <cstdio>
#include <cstring>
#include <algorithm>
#include <vector>
#define INF 0x3f3f3f3f
#define MOD 1000000007
using namespace std;
typedef long long LL;

const int maxn = 2e5 + 10;
int ans[maxn];
int cnt[maxn];
int num[maxn];
vector<int> G[maxn];

int nxt(int x) {
    int up = x - 3, down = x % 3;
    while (up > down) {
        if (cnt[up]) return up;
        up -= 3;
    }
    return down;
}

int main() {
    int N;
    scanf("%d", &N);
    for (int i = 1; i <= N; i++) {
        scanf("%d", &num[i]);
        G[num[i]].push_back(i);
        cnt[num[i]]++;
    }
    bool flag = 1;
    int k = 0;    
    while (1) {
        if (ans[0] == N) break;
        if (cnt[k]) {
            flag = 1;
            int tmp = G[k][--cnt[k]];
            ans[++ans[0]] = tmp;
            k++;
        } else {
            k = nxt(k);
            if (flag == 0) break;
            flag = 0;
        }
    }
    if (ans[0] == N) {
        puts("Possible");
        for (int i = 1; i <= ans[0]; i++) {
            printf("%d%c", ans[i], i == ans[0] ? '
' : ' ');
        }
    } else {
        puts("Impossible");
    }
    return 0;
}