Submission #236413

#	Time	Username	Problem	Language	Result	Execution time	Memory
236413		kaplanbar	Three Friends (BOI14_friends)	C++17	100 / 100	167 ms	54216 KiB

friends

// In the name of God

#include <bits/stdc++.h>

using namespace std;

const int N = 2e6+5;

const int mod = 1e9+7;
const long long p = 37;

long long pw[N], invpw[N];

long long fast_pw(long long x, long long y) {
	long long ret = 1;
	for(;y;y>>=1) {
		if(y & 1) ret = (ret * x) % mod;
		x = (x * x) % mod;
	}
	return ret;
}

int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(0);

	pw[0] = 1;

	invpw[0] = 1;
	invpw[1] = fast_pw(p, mod-2);

	for(int i = 1; i < N; i++) pw[i] = (pw[i-1] * p) % mod;

	for(int i = 2; i < N; i++) invpw[i] = (invpw[i-1] * invpw[1]) % mod;

	int n;
	cin >> n;
	
	string s;
	cin >> s;

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

	int ans = 0;

	vector<long long> hs(n, 0);

	for(int i = 0; i < n; i++) {
		hs[i] = ((s[i] - 'A' + 1) * pw[i]) % mod;
		if(i) hs[i] = (hs[i] + hs[i-1]) % mod;
	}


	auto find_hs = [&](int l, int r) {
		// returns the hash of (l, r)
		long long ret = hs[r];
		if(l) ret = (ret - hs[l - 1] + mod) % mod;
		ret = (ret * invpw[l]) % mod;
		return ret;
	};

	auto find_hs_x = [&](int l, int r, int x) {
		//returns hash of (l, r) except character x
		if(x < l || x > r) return find_hs(l, r);
		long long ret = x != l ? find_hs(l, x - 1) : 0;
		long long right = x != r ? find_hs(x+1, r) : 0;

		ret = (ret + (pw[x-l] * right) % mod ) % mod;
		return ret;
	};

	unordered_set<int> se;

	for(int i = 0; i < n; i++) {
		long long left,right;
		if(i < n / 2) {
			left = find_hs_x(0, n / 2, i);
			right = find_hs(n / 2 + 1, n - 1);
		}
		else if(i == n / 2) {
			left = find_hs(0, n / 2  - 1);
			right = find_hs(n / 2 + 1, n - 1);
		}
		else {
			left = find_hs(0, n / 2 - 1);
			right = find_hs_x(n / 2, n - 1, i);
		}
		if(left == right) {
			se.insert(find_hs_x(0, n-1, i));
		}
	}

	if(se.size() == 0) cout << "NOT POSSIBLE";
	if(se.size() > 1) cout << "NOT UNIQUE";
	if(se.size() == 1) {
		for(int i = 0; i < n; i++) {
			long long left,right;
			if(i < n / 2) {
				left = find_hs_x(0, n / 2, i);
				right = find_hs(n / 2 + 1, n - 1);
			}
			else if(i == n / 2) {
				left = find_hs(0, n / 2  - 1);
				right = find_hs(n / 2 + 1, n - 1);
			}
			else {
				left = find_hs(0, n / 2 - 1);
				right = find_hs_x(n / 2, n - 1, i);
			}
			if(left == right) {
				string ans = "";
				int pos = 0;
				while(ans.size()!=n/2) {
					if(pos==i) {
						pos++;
						continue;
					}
					ans += s[pos];
					pos++;
				}
				cout << ans;
				exit(0);
			}
		}
	}

	return 0;
}

Compilation message (stderr)

friends.cpp: In function 'int main()':
friends.cpp:116:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     while(ans.size()!=n/2) {
           ~~~~~~~~~~^~~~~
friends.cpp:47:6: warning: unused variable 'ans' [-Wunused-variable]
  int ans = 0;
      ^~~

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