PKU 1106 Transmitters

• 幾何
• 複素数ライブラリを使うと楽
• 角度でソートして尺取メソッド

int main() {
	double r;
	for (int X, Y; cin >> X >> Y >> r && r >= 0.0; ) {
		int N;
		cin >> N;
		vector<double> angles;
		for (int n = 0; n < N; ++n) {
			int x, y;
			cin >> x >> y;
			x -= X;
			y -= Y;
			if (sqrt((double)x * x + y * y) < r + EPS) {
				angles.push_back(atan2((double)y, (double)x));
			}
		}

		const int M = angles.size();
		const int M2 = M * 2;
		sort(angles.begin(), angles.end());

		for (int m = 0; m < M; ++m) {
			angles.push_back(angles[m] + PI2);
		}

		int bestAnswer = 0;
		int end = 0;
		for (int m = 0; m < M; ++m) {
			while (end < M2 && angles[end] < angles[m] + PI + EPS) {
				++end;
			}
			bestAnswer = max(bestAnswer, end - m);
		}

		cout << bestAnswer << endl;
	}
}