Submission #135651

#	Time	Username	Problem	Language	Result	Execution time	Memory
135651		tdwn	Three Friends (BOI14_friends)	C++17	35 / 100	681 ms	35672 KiB

friends

#include <bits/stdc++.h>
#define ll long long 
#define pb push_back
#define mp make_pair
using namespace std;
const int maxn = 2001000;
const ll hashmod = 1e9 + 9;
const ll k = 37LL;
ll pref_hash[maxn], n;
ll inv[maxn];
string str;
 
inline ll power(ll a, ll b) {
	if(b == 0) return 1LL;
	else if(b == 1) return 1LL * a;
	else {
		if(b % 2 == 0) {
			ll temp = power(a, b/2);
			return (temp*temp)%hashmod;
		}
		else return (a*power(a, b-1))%hashmod;
	}
}
 
inline ll get_hash(int l, int r) {
	ll diff = pref_hash[r] + hashmod - pref_hash[l-1];
	if(diff > hashmod) diff -= hashmod;
	diff *= inv[l-1];
	//while(diff > hashmod) diff -= hashmod;
	diff %= hashmod;
	return diff;
}
 
int main() {
	ios_base::sync_with_stdio(false);
	cin>>n;
	cin>>str;
 
	if(n % 2 == 0) {
		cout<<"NOT POSSIBLE\n";
		return 0;
	}
 
	ll kk = k;
	inv[0] = 1LL;
	for(int i=1;i<=n;i++) {
		pref_hash[i] = pref_hash[i-1] + ((str[i-1]-'a'+1)*kk)%hashmod;
		//if(pref_hash[i] > hashmod) pref_hash[i] -= hashmod;
		pref_hash[i] %= hashmod;
		inv[i] = power(kk, hashmod-2);
		kk *= k; kk %= hashmod;
	}
 
	//cout<<get_hash(1, 2)<<" and "<<get_hash(5, 6)<<"\n";
 
	ll half_sz = n / 2;
	ll mid = (n+1)/2;
	ll sz = n / 2;

	bool dwn = false, upp = false;

	vector<int>v;
	for(int i=1;i<=n;i++) {
		if(i == 1 && get_hash(2, 1+sz) == get_hash(2+sz, n)) {
			if(i <= mid) dwn = true;
			else upp = true;
			v.pb(i);
		}
		else if(i == n && get_hash(1, sz) == get_hash(sz+1, n-1)) {v.pb(i); if(i<=mid) dwn = true; else upp = true;}
		else if(i == mid && get_hash(1, sz) == get_hash(mid+1, n)){v.pb(i); if(i<=mid) dwn = true; else upp = true;}
		else if(i < mid) {
			ll lft_sz = i-1;
			ll rgh_sz = mid - i;
 

			if(get_hash(1, lft_sz) == get_hash(mid+1, mid+lft_sz) && get_hash(i+1, i+rgh_sz) == get_hash(n-rgh_sz+1, n)){v.pb(i); if(i<=mid) dwn = true; else upp = true;}
		}
		else if(i > mid){
			ll lft_sz = i - mid;
			ll rgh_sz = n - i;
 
			if(get_hash(1, lft_sz) == get_hash(mid, mid+lft_sz-1) && get_hash(lft_sz+1, mid-1) == get_hash(n - rgh_sz + 1, n)) {v.pb(i); if(i<=mid) dwn = true; else upp = true;}
		}
	}
 
	if(v.size() == 0) {
		cout<<"NOT POSSIBLE\n";
		return 0;
	}
	
	if(v.size() == 1) {
		for(int i:v) {
			if(i == 1) cout<<str.substr(mid, half_sz); //hashes.insert(get_hash(mid+1, n));
			else if(i == n) cout<<str.substr(0, half_sz); //hashes.insert(get_hash(1, mid));
			else if(i == mid) cout<<str.substr(mid, half_sz); //hashes.insert(get_hash(n-half_sz+1, n));
			else if(i < mid) cout<<str.substr(mid, half_sz); //hashes.insert(get_hash(n-half_sz+1, n));
			else if(i > mid) cout<<str.substr(0, half_sz); //hashes.insert(get_hash(1, mid));
			return 0;
		}
	}

	if(upp && dwn && get_hash(1, mid-1) != get_hash(mid+1, n)) {
		cout<<"NOT UNIQUE\n";
	}
	else {
		for(int i:v) {
			if(i == 1) cout<<str.substr(mid, half_sz); //hashes.insert(get_hash(mid+1, n));
			else if(i == n) cout<<str.substr(0, half_sz); //hashes.insert(get_hash(1, mid));
			else if(i == mid) cout<<str.substr(mid, half_sz); //hashes.insert(get_hash(n-half_sz+1, n));
			else if(i < mid) cout<<str.substr(mid, half_sz); //hashes.insert(get_hash(n-half_sz+1, n));
			else if(i > mid) cout<<str.substr(0, half_sz); //hashes.insert(get_hash(1, mid));
			return 0;
		}
	}
}

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