Submission #679625

#	Time	Username	Problem	Language	Result	Execution time	Memory
679625		peijar	Three Friends (BOI14_friends)	C++17	35 / 100	1071 ms	104232 KiB

friends

#include <bits/stdc++.h>
#define int long long
using namespace std;

namespace std {
template <typename T> ostream &operator<<(ostream &out, const vector<T> &vec) {
  out << "[";
  for (int i = 0; i < (int)vec.size(); ++i) {
    out << vec[i];
    if (i + 1 < (int)vec.size())
      out << ", ";
  }
  return out << "]";
}
} // namespace std

void dbg_out() { cout << endl; }
template <typename Head, typename... Tail> void dbg_out(Head H, Tail... T) {
  cout << ' ' << H;
  dbg_out(T...);
}

#ifdef DEBUG
#define dbg(...) cout << "(" << #__VA_ARGS__ << "):", dbg_out(__VA_ARGS__)
#else
#define dbg(...)
#endif

/*
 * sa[i] = starting index of i-th smallest suffix
 * sa.len() = n+ 1, sa[0] = n
 * lcp[i] = lcp(sa[i], sa[i-1]), lcp[0] = 0, len(sa) = n+1
 */

template <class T> struct RMQ {
  vector<vector<T>> jmp;
  RMQ(const vector<T> &V) : jmp(1, V) {
    for (int pw = 1, k = 1; pw * 2 <= (int)V.size(); pw *= 2, ++k) {
      jmp.emplace_back((int)V.size() - pw * 2 + 1);
      for (int j = 0; j < (int)jmp[k].size(); ++j)
        jmp[k][j] = min(jmp[k - 1][j], jmp[k - 1][j + pw]);
    }
  }
  T query(int a, int b) { // [a, b)
    assert(a < b);
    int dep = 31 - __builtin_clz(b - a);
    return min(jmp[dep][a], jmp[dep][b - (1 << dep)]);
  }
};

struct SuffixArray {
  vector<int> sa, lcp, rank;
  string s;
  RMQ<int> rmq;

  SuffixArray(string &_s, int lim = 256) : rmq({}) {
    s = _s;
    int n = s.size() + 1, k = 0, a, b;
    vector<int> x(s.begin(), s.end() + 1), y(n), ws(max(n, lim));
    sa = lcp = rank = y, iota(sa.begin(), sa.end(), 0);

    for (int j = 0, p = 0; p < n; j = max(1LL, 2 * j), lim = p) {
      p = j, iota(y.begin(), y.end(), n - j);
      for (int i = 0; i < n; ++i)
        if (sa[i] >= j)
          y[p++] = sa[i] - j;
      fill(ws.begin(), ws.end(), 0);
      for (int i = 0; i < n; ++i)
        ws[x[i]]++;
      for (int i = 1; i < lim; ++i)
        ws[i] += ws[i - 1];
      for (int i = n; i--;)
        sa[--ws[x[y[i]]]] = y[i];
      swap(x, y);
      p = 1, x[sa[0]] = 0;
      for (int i = 1; i < n; ++i)
        a = sa[i - 1], b = sa[i],
        x[b] = (y[a] == y[b] and y[a + j] == y[b + j]) ? p - 1 : p++;
    }
    for (int i = 1; i < n; ++i)
      rank[sa[i]] = i;
    for (int i = 0, j; i < n - 1; lcp[rank[i++]] = k)
      for (k &&k--, j = sa[rank[i] - 1]; s[i + k] == s[j + k]; k++)
        ;
    rmq = RMQ<int>(lcp);
  }

  int getLCP(int i, int j) {
    if (i == j)
      return s.size() - i;
    i = rank[i];
    j = rank[j];
    if (i > j)
      swap(i, j);
    return rmq.query(i + 1, j + 1);
  }
};

bool get(string U) {
  int N = U.size() / 2;
  SuffixArray suffix(U);
  for (int skip = N; skip < 2 * N + 1; ++skip) {
    if (suffix.getLCP(0, N) < skip - N)
      continue;
    if (skip < 2 * N and suffix.getLCP(skip - N, skip + 1) < N - (skip - N))
      continue;
    return true;
  }
  return false;
}

signed main(void) {
  ios_base::sync_with_stdio(false);
  cin.tie(0);

  int N;
  string U;
  cin >> N >> U;

  if (N % 2 == 0) {
    cout << "NOT POSSIBLE\n";
    return 0;
  }

  string T = U;
  reverse(T.begin(), T.end());
  bool okL = get(U);
  bool okR = get(T);
  N /= 2;
  string L = U.substr(0, N);
  string R = U.substr(N + 1, N);
  dbg(L, R);
  dbg(okL, okR);

  if (!okL and !okR) {
    cout << "NOT POSSIBLE\n";
    return 0;
  }
  if (okL and okR and L != R) {
    cout << "NOT UNIQUE\n";
    return 0;
  }
  cout << (okL ? L : R) << endl;
}

• Subtask 1: 35 / 35
• Subtask 2: 0 / 65