Language: C++

// see https://github.com/LumaKernel/vimfiles/tree/master/snippets/cp-cpp/temp.snip
/// --- .lumrc Template {{{ ///
#if 0
#define assert(...)
#endif
#include <bits/stdc++.h>
using namespace std;

using ll = long long;
using ull = unsigned long long;
using ld = long double;
using P = tuple<ll, ll>;
using P3 = tuple<ll, ll, ll>;
using VI = vector<int>;
using VL = vector<ll>;
using VVI = vector<VI>;
using VVL = vector<VL>;
using VP = vector<P>;
using VS = vector<string>;

#define omajinai ios::sync_with_stdio(false);cin.tie(0)

#define FOR(i,a,b) for(int i=int(a);i<int(b);++i)
#define FORI(i,a,b) for(int i=int(a);i<=int(b);++i)
#define REP(i,n) FOR(i,0,n)
#define REPI(i,n) FORI(i,0,n)
#define RFOR(i,a,b) for(int i=int(b)-1;i>=int(a);--i)
#define RFORI(i,a,b) for(int i=int(b);i>=int(a);--i)
#define RREP(i,n) RFOR(i,0,n)
#define RREPI(i,n) RFORI(i,0,n)

#define ALL(a) (a).begin(),(a).end()
#define UNIQUE(a) (a).erase(unique(ALL(a)),(a).end())
#define PB push_back
#define EACH(i,c) REP(i,(c).size())
#define REACH(i,c) RREP(i,(c).size())
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort(ALL(c))

#define BR cout<<"\n";

#ifdef DEBUG
#define dump(...) cerr<<"["<<__LINE__<< "] "<<#__VA_ARGS__<<" = "<<make_tuple(__VA_ARGS__)<<"\n";
#else
#define dump(...)
#endif

#define YES(x) cout<<((x)?"YES":"NO")<<"\n";
#define Yes(x) cout<<((x)?"Yes":"No")<<"\n";
#define yes(x) cout<<((x)?"yes":"no")<<"\n";

inline int omajinai_int_in(){omajinai;int n;cin>>n;return n;}
inline ll omajinai_ll_in(){omajinai;ll n;cin>>n;return n;}
inline string omajinai_string_in(){omajinai;string n;cin>>n;return n;}
inline int int_in(){int n;cin>>n;return n;}
inline ll ll_in(){ll n;cin>>n;return n;}
inline string string_in(){string n;cin>>n;return n;}
#define oini omajinai_int_in()
#define oinl omajinai_ll_in()
#define oins omajinai_string_in()
#define ini int_in()
#define inl ll_in()
#define ins string_in()

#define isInside(y,x) (0<=(y)&&(y)<h&&0<=(x)&&(x)<w)

#define fi(x) (get<0>(x))
#define se(x) (get<1>(x))
#define th(x) (get<2>(x))
#define fo(x) (get<3>(x))
#define fif(x) (get<4>(x))

template <typename T> ostream &operator<<(ostream &o, const vector<T> &v) { o << '{'; EACH(i, v) o << v[i] << (i != v.size()-1 ? ", " : ""); o << "}";  return o; }

template<int n, class...T> typename enable_if<(n>=sizeof...(T))>::type _ot(ostream &, tuple<T...> const &){}
template<int n, class...T> typename enable_if<(n< sizeof...(T))>::type _ot(ostream & os, tuple<T...> const & t){ os << (n==0?"":", ") << get<n>(t); _ot<n+1>(os, t); }
template<class...T> ostream & operator<<(ostream & os, tuple<T...> const & t){ _ot<0>(os, t); return os; }

template <typename T> inline void smax(T &a, T b) { a = a > b ? a : b; }
template <typename T> inline void smin(T &a, T b) { a = a < b ? a : b; }

template<typename T> int log2max(T x) {
  int res = 0;
  ll tmp = 1;
  while(tmp < x) tmp <<= 1, res++;
  return res;
}

constexpr double PI = acos(-1);

/// }}}--- ///

constexpr int INF = 1e9+1;
constexpr ll LINF = 1e18+1;
constexpr int MOD = 1e9+7;

/// --- Lowlink Library {{{ ///

// undirected graph
// count edges
// construct : (int size)
// do : calc()
struct Lowlink {
  struct Edge {
    int to, idx;
    Edge(int to, int idx): to(to), idx(idx) {}
  };
  int size;
  int edgeSize = 0;
  vector< vector< Edge > > g;
  VI ord, low;

  VI used;
  VI isBridge, isArticulation;
  Lowlink(int n): size(n), g(n), ord(n), low(n) {}

  void addEdge(int a, int b) {
    g[a].emplace_back(b, edgeSize);
    g[b].emplace_back(a, edgeSize);
    edgeSize++;
  }

  void calc() {
    used.assign(size, 0);
    isBridge.assign(edgeSize, 0);
    isArticulation.assign(size, 0);

    int k = 0;
    dfs(0, -1, k);
  }

  // p just means "just before"
  private :
  void dfs(int i, int p, int &k) {
    used[i] = 1;
    ord[i]=low[i]=k++;
    isArticulation[i] = 0;
    int DFSTreeDegree = 0;
    for(Edge edge : g[i]) {
      int j = edge.to;
      int idx = edge.idx;
      if(used[j]) {
        if(j != p) {
          // back edge
          smin(low[i], ord[j]);
        }
      } else {
        // on dfs-tree
        DFSTreeDegree++;
        dfs(j, i, k);
        smin(low[i], low[j]);
        isBridge[idx] = ord[i] < low[j];
        if(p != -1 && ord[i] <= low[j]) {
          isArticulation[i] = 1;
        }
      }
    }
    if(p == -1 && DFSTreeDegree > 1) {
      isArticulation[i] = 1;
    }
  }
};

/// }}}--- ///

int n=ini;
Lowlink lowlink(n);
VI onCircuit(n, 0);
VI used(n, 0);
VVI orig(n);
VI rev(n);
VI num(n, 0);

void dfs(int i) {
  if(onCircuit[i]) return;
  if(used[i]) return;
  used[i] = 1;
  set<int> st;
  for(int j : orig[i]) {
    assert(onCircuit[j] == false);
    dfs(j);
    st.insert(num[j]);
  }
  int t=0;
  while(st.count(t)) t++;
  num[i] = t;
}

bool solve(int now) {
  int last = now;
  now = rev[now];
  while(1) {
    set<int> st;
    for(int j:orig[now]) {
      st.insert(num[j]);
    }
    int nnum = 0;
    while(st.count(nnum)) nnum++;
    if(num[now] <= nnum) {
      if(num[now] == nnum) {
        if(last == now) {
          return 1;
        }
      } else {
        num[now] = nnum;
        last = now;
      }
    } else {
      return 0;
    }
    now = rev[now];
  }
}

int main() {
  REP(i, n) {
    int p = ini-1;
    // p -> i
    lowlink.addEdge(p, i);
    orig[p].PB(i);
    rev[i] = p;
  }
  lowlink.calc();

  REP(i, n) {
    for(auto edge : lowlink.g[i]) {
      int j = edge.to, idx = edge.idx;
      if(!lowlink.isBridge[idx]) {
        onCircuit[i] = 1;
      }
    }
  }

  REP(i, n) dfs(i);

  int now=-1;
  REP(i, n) {
    if(onCircuit[i]) {
      now = i;
    }
  }
  assert(now != -1);
  if(solve(now)) {
    cout << "POSSIBLE" << endl;
    return 0;
  }
  REP(i, 10) {
    REP(i, n) {
      if(onCircuit[i]) {
        num[i] = 0;
      }
    }
    now = rev[now];
    if(solve()) {
      cout << "POSSIBLE" << endl;
      return 0;
    }
  }
  cout << "IMPOSSIBLE" << endl;
}

//


Compiler output:
a.cc: In function 'int main()':
a.cc:254:13: error: too few arguments to function 'bool solve(int)'
  254 |     if(solve()) {
      |        ~~~~~^~
a.cc:189:6: note: declared here
  189 | bool solve(int now) {
      |      ^~~~~