Submission #107577

#	Time	Username	Problem	Language	Result	Execution time	Memory
107577		dfistric	Three Friends (BOI14_friends)	C++14	100 / 100	191 ms	66980 KiB

friends

#include <bits/stdc++.h>

#define FOR(i, a, b) for (int i = (a); i < (b); i++)
#define FORd(i, a, b) for (int i = (a); i >= (b); i--)
#define REP(i, n) FOR(i, 0, n)
#define ll long long

using namespace std;

const int MAXN = 2001000;
ll ha[MAXN], pot[MAXN];
ll ha2[MAXN], pot2[MAXN];
const int BASE = 35537;
const int MOD = 1e9 + 7;
const int BASE2 = 57737;
const int MOD2 = 1e9 + 9;

ll val(int x, int y) {
    if (y < x) return 0;

    ll out = ha[y];
    if (x) out = (((out - ha[x - 1] * pot[y - x + 1]) % MOD) + MOD) % MOD;
    return out;
}

ll val2(int x, int y) {
    if (y < x) return 0;

    ll out = ha2[y];
    if (x) out = (((out - ha2[x - 1] * pot2[y - x + 1]) % MOD2) + MOD2) % MOD2;
    return out;
}

int main() {
    ios_base::sync_with_stdio(false);
    pot[0] = 1;
    FOR(i, 1, MAXN) pot[i] = (pot[i - 1] * BASE) % MOD; 
    pot2[0] = 1;
    FOR(i, 1, MAXN) pot2[i] = (pot2[i - 1] * BASE2) % MOD2; 
 
    int n;
    cin >> n;
    string s;
    cin >> s;

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

    ll curr = 0, curr2 = 0;
    REP(i, n) {
        curr = (curr * BASE + s[i]) % MOD;
        ha[i] = curr;
        curr2 = (curr2 * BASE2 + s[i]) % MOD2;
        ha2[i] = curr2;
    }

    string out = "";
    ll V = -1, V2 = -1;
    REP(i, n) {
        if (i <= n / 2) {
            ll a = (val(0, i - 1) * pot[n / 2 - i] + val(i + 1, n / 2)) % MOD;
            ll b = val(n / 2 + 1, n - 1);
            ll a2 = (val2(0, i - 1) * pot2[n / 2 - i] + val2(i + 1, n / 2)) % MOD2;
            ll b2 = val2(n / 2 + 1, n - 1);

            if (a != b) continue;
            if (a2 != b2) continue;
            
            if (out != "" && (V != a || V2 != a2))  {
                cout << "NOT UNIQUE\n";
                return 0;
            } else if (out == "") {
                V = a;
                V2 = a2;
                out = s.substr(n / 2 + 1, n / 2);
            }
        } else {
            ll a = val(0, n / 2 - 1);
            ll b = (val(n / 2, i - 1) * pot[n - i - 1] + val(i + 1, n - 1)) % MOD;
            ll a2 = val2(0, n / 2 - 1);
            ll b2 = (val2(n / 2, i - 1) * pot2[n - i - 1] + val2(i + 1, n - 1)) % MOD2;

            if (a != b) continue;
            if (a2 != b2) continue;

            if (out != "" && (V != a || V2 != a2))  {
                cout << "NOT UNIQUE\n";
                return 0;
            } else if (out == "") {
                V = a;
                V2 = a2;
                out = s.substr(0, n / 2);
            }
        }
    }

    if (out == "") cout << "NOT POSSIBLE\n";
    else           cout << out << "\n";

    return 0;
}

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