Submission #26469

#	Time	Username	Problem	Language	Result	Execution time	Memory
26469		zscoder	Three Friends (BOI14_friends)	C++14	35 / 100	356 ms	20908 KiB

friends

#include <bits/stdc++.h>

using namespace std;

#define fi first
#define se second
#define mp make_pair
#define pb push_back

typedef long long ll;
typedef pair<int,int> ii;
typedef vector<int> vi;
typedef long double ld; 

const int INF = 1e9 + 7;
const int MOD = 1e9 + 9;
const int C = 2017;

ll modpow(ll a, ll b)
{
	if(b<0) b+=(MOD-1);
	ll r= 1;
	while(b)
	{
		if(b&1) r = (r*a)%MOD;
		a=(a*a)%MOD;
		b>>=1;
	}
	return r;
}

ll inv(ll x)
{
	return modpow(x, MOD - 2);
}

ll invC;

ll dp[2000001];

ll hsh(int l, int r)
{
	if(r<l) return 0;
	r++; l++;
	return ((dp[r] - dp[l-1])%MOD+MOD)%MOD;
}

int main()
{
	ios_base::sync_with_stdio(0); cin.tie(0);
	//freopen("friend.in", "r", stdin);
	invC = inv(C);
	int n; string s;
	cin >> n >> s;
	if(n%2==0)
	{
		cout << "NOT POSSIBLE";
		return 0;
	}
	int l = n/2;
	vi pos;
	//string a = s.substr(0, l);
	//string b = s.substr(l, l);
	//if(a == b) pos = n - 1;
	bool started = false;
	bool flag = true; char theletter = '$';
	ll CMOD = modpow(C,l+1);
	for(int i = 1; i <= n; i++)
	{
		dp[i] = (dp[i-1] + ll(s[i-1]-'A'+1)*modpow(C, n - i))%MOD;
	}
	for(int i = 0; i < n; i++) //it's placed at pos i
	{
		if(started)
		{
			if(s[i] != theletter) flag = false;
		}
		if(i < l)
		{
			ll h1 = hsh(0,i-1);
			ll h2 = hsh(i+1,l);
			h2 = (h2*C)%MOD;
			h1 += h2;
			h1%=MOD;
			if(h1 == (hsh(l+1,n-1)*CMOD)%MOD)
			{
				pos.pb(i);
				if(!flag)
				{
					cout << "NOT UNIQUE";
					return 0;
				}
				if(!started)
				{
					started = true;
					theletter = s[i];
				}
			}
		}
		if(i == l)
		{
			if(hsh(0,l-1)==(hsh(l+1,n-1)*CMOD)%MOD)
			{
				pos.pb(i);
				if(!flag)
				{
					cout << "NOT UNIQUE";
					return 0;
				}
				if(!started)
				{
					started = true;
					theletter = s[i];
				}
			}
		}
		if(i > l)
		{
			ll h1 = hsh(l,i-1);
			ll h2 = hsh(i+1,n-1);
			h1=(h1*invC)%MOD;
			h1=(h1+h2)%MOD;
			if(hsh(0,l-1)==(h1*CMOD)%MOD)
			{
				pos.pb(i);
				if(!flag)
				{
					cout << "NOT UNIQUE";
					return 0;
				}
				if(!started)
				{
					started = true;
					theletter = s[i];
				}
			}
		}
	}
	if(pos.empty())
	{
		cout << "NOT POSSIBLE"; return 0;
	}
	/*
	char commonletter = s[pos[0]];
	for(int i = 0; i < pos.size(); i++)
	{
		if(s[pos[i]] != commonletter)
		{
			cout << "NOT POSSIBLE"; return 0;
		}
	}
	*/
	int cnt = 0;
	for(int i = 0; i < pos[0]; i++)
	{
		if(cnt >= l) break;
		cout << s[i];
		cnt++;
	}
	for(int i = pos[0] + 1; i < n; i++)
	{
		if(cnt >= l) break;
		cout << s[i];
		cnt++;
	}
}

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