Submission #135574

#	Time	Username	Problem	Language	Result	Execution time	Memory
135574		tdwn	Three Friends (BOI14_friends)	C++17	0 / 100	199 ms	10992 KiB

friends

#include <bits/stdc++.h>
#define ll long long 
#define pb push_back
#define mp make_pair
using namespace std;
const int maxn = 200100;
const ll hashmod = 1e9 + 9;
const ll k = 37LL;
ll pref_hash[maxn], n;
ll inv[maxn];
string str;

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;
	}
}

ll get_hash(int l, int r) {
	ll diff = pref_hash[r] + hashmod - pref_hash[l-1];
	diff %= hashmod;
	diff *= inv[l-1];
	diff %= hashmod;
	return diff;
}

int main() {
	cin>>n;
	cin>>str;

	//ofstream fout("output.out");

	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;
		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;
	vector<int>v;
	for(int i=1;i<=n;i++) {
		if(i == 1 && get_hash(2, 1+sz) == get_hash(2+sz, n)) v.pb(i);
		else if(i == n && get_hash(1, sz) == get_hash(sz+1, n-1)) v.pb(i);
		else if(i == mid && get_hash(1, sz) == get_hash(mid, n)) v.pb(i);
		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);
		}
		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(v.size() == 0) {
		cout<<"NOT POSSIBLE\n";
		return 0;
	}
	else if(v.size() > 1) {
		cout<<"NOT UNIQUE\n";
		return 0;
	}

	for(int i:v) {
		//cout<<i<<"\n";
		if(i == 1)
			for(int j=mid+1;j<=n;j++)
				cout<<str[j-1];
		else if(i == n) 
			for(int j=1;j<mid;j++)
				cout<<str[j-1];
		else if(i == mid) 
			for(int j=n-half_sz+1;j<=n;j++)
				cout<<str[j-1];
		else if(i < mid) {
			//cout<<n-half_sz+1<<"\n";
			for(int j=n-half_sz+1;j<=n;j++) 
				cout<<str[j-1];
		}
		else if(i > mid)
			for(int j=1;j<mid;j++)
				cout<<str[j-1];
	}
}

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