Submission #78671

#	Time	Username	Problem	Language	Result	Execution time	Memory
78671		thebes	Three Friends (BOI14_friends)	C++14	100 / 100	166 ms	38732 KiB

friends

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

const int MN = 2e6+6;
int h[MN], pw[MN]={1}, N, L, i, cnt;
string s, ans; set<int> lol;
int hsh(int l,int r){return h[r]-h[l-1]*pw[r-l+1];}
int main(){
    cin >> N >> s;
    for(i=1;i<=N;i++){
        h[i]=h[i-1]*131+s[i-1]-'A';
        pw[i]=pw[i-1]*131;
    }
    if(N%2){
        L = N/2;
        for(i=1;i<=N;i++){
            if(i<L+1){
                int l1 = i-1;
                int h1 = hsh(1,l1)*pw[L-l1]+hsh(i+1,i+L-l1);
                int h2 = hsh(N-L+1,N);
                if(h1==h2){
                    cnt++; lol.insert(hsh(N-L+1,N));
                    if(cnt == 1){
                        for(int j=N-L+1;j<=N;j++)
                            ans.push_back(s[j-1]);
                    }
                }
            }
            else if(i>L+1){
                int h1 = hsh(1,L);
                int h2 = hsh(L+1,i-1)*pw[N-i]+hsh(i+1,N);
                if(h1==h2){
                    cnt++; lol.insert(hsh(1,L));
                    if(cnt == 1){
                        for(int j=1;j<=L;j++)
                            ans.push_back(s[j-1]);
                    }
                }
            }
            else{
                if(hsh(1,i-1)==hsh(i+1,N)){
                    cnt++; lol.insert(hsh(1,L));
                    if(cnt == 1){
                        for(int j=1;j<=L;j++)
                            ans.push_back(s[j-1]);
                    }
                }
            }
        }
        if(cnt==0) printf("NOT POSSIBLE\n");
        else if(lol.size()>1) printf("NOT UNIQUE\n");
        else printf("%s\n",ans.c_str());
    }
    else printf("NOT POSSIBLE\n");
    return 0;
}

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