Submission #19550853

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Source Code Expand

#include <iostream>
#include <atcoder/all>
using namespace std;
using namespace atcoder;

using mint = modint1000000007;

int main() {
	int N;
	string S;
	cin >> N >> S;
	N *= 2;
	if (S.front() == 'W' || S.back() == 'W') {
		cout << 0 << endl;
		return 0;
	}
	vector<int>LR(N, 0);
	LR.back() = 1;
	int right = 1;
	for (int i = 1; i < N - 1; i++) {
		LR[i] = (S[i] == S[i - 1] ? 1 - LR[i - 1] : LR[i - 1]);
		right += LR[i];
	}
	if (right != N / 2) {
		cout << 0 << endl;
		return 0;
	}

	mint ans = 1;
	int num = 0;
	for (int i = 0; i < N; i++) {
		if (LR[i]) {
			ans *= num;
			num--;
		}
		else num++;
	}
	for (int i = 1; i <= N / 2; i++) ans *= i;
	cout << ans.val() << endl;
}

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