Submission #484303

#	Time	Username	Problem	Language	Result	Execution time	Memory
484303		rainboy	Planine (COCI21_planine)	C11	110 / 110	280 ms	47556 KiB

Main

#include <stdio.h>

#define N	1000000

unsigned int X = 12345;

int rand_() {
	return (X *= 3) >> 1;
}

long long cross2(long long x1, long long y1, long long x2, long long y2) {
	return x1 * y2 - x2 * y1;
}

int xx[N], yy[N];

long long cross(int i, int j, int k) {
	return cross2(xx[j] - xx[i], yy[j] - yy[i], xx[k] - xx[i], yy[k] - yy[i]);
}

long long pp[N], qq[N];

void sort(int *ii, int l, int r) {
	while (l < r) {
		int i = l, j = l, k = r, i_ = ii[l + rand_() % (r - l)], tmp;

		while (j < k) {
			long long c = cross2(pp[ii[j]], qq[ii[j]], pp[i_], qq[i_]);

			if (c == 0)
				j++;
			else if (c < 0) {
				tmp = ii[i], ii[i] = ii[j], ii[j] = tmp;
				i++, j++;
			} else {
				k--;
				tmp = ii[j], ii[j] = ii[k], ii[k] = tmp;
			}
		}
		sort(ii, l, i);
		l = k;
	}
}

int main() {
	static int ll[N], rr[N], qu[N], ii[N];
	int n, y, i, k, cnt;
	long long p, q;

	scanf("%d%d", &n, &y);
	for (i = 0; i < n; i++)
		scanf("%d%d", &xx[i], &yy[i]);
	cnt = 0;
	for (i = 0; i < n; i++) {
		while (cnt >= 2 && cross(qu[cnt - 2], qu[cnt - 1], i) >= 0)
			cnt--;
		ll[i] = cnt ? qu[cnt - 1] : -1;
		qu[cnt++] = i;
	}
	cnt = 0;
	for (i = n - 1; i >= 0; i--) {
		while (cnt >= 2 && cross(qu[cnt - 2], qu[cnt - 1], i) <= 0)
			cnt--;
		rr[i] = cnt ? qu[cnt - 1] : -1;
		qu[cnt++] = i;
	}
	for (i = 2; i < n - 1; i += 2) {
		pp[i] = cross2(xx[ll[i]], yy[ll[i]] - y, xx[i] - xx[ll[i]], yy[i] - yy[ll[i]]);
		qq[i] = cross2(1, 0, xx[i] - xx[ll[i]], yy[i] - yy[ll[i]]);
		if (qq[i] < 0)
			pp[i] = -pp[i], qq[i] = -qq[i];
		pp[i + 1] = cross2(xx[rr[i]], yy[rr[i]] - y, xx[i] - xx[rr[i]], yy[i] - yy[rr[i]]);
		qq[i + 1] = cross2(1, 0, xx[i] - xx[rr[i]], yy[i] - yy[rr[i]]);
		if (qq[i + 1] < 0)
			pp[i + 1] = -pp[i + 1], qq[i + 1] = -qq[i + 1];
		ii[i / 2 - 1] = i + 1;
	}
	sort(ii, 0, n / 2 - 1);
	p = -1, q = 0, k = 0;
	for (i = 0; i < n / 2 - 1; i++)
		if (cross2(p, q, pp[ii[i] - 1], qq[ii[i] - 1]) < 0)
			p = pp[ii[i]], q = qq[ii[i]], k++;
	printf("%d\n", k);
	return 0;
}

Compilation message (stderr)

Main.c: In function 'main':
Main.c:50:2: warning: ignoring return value of 'scanf' declared with attribute 'warn_unused_result' [-Wunused-result]
   50 |  scanf("%d%d", &n, &y);
      |  ^~~~~~~~~~~~~~~~~~~~~
Main.c:52:3: warning: ignoring return value of 'scanf' declared with attribute 'warn_unused_result' [-Wunused-result]
   52 |   scanf("%d%d", &xx[i], &yy[i]);
      |   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~

• Subtask 1: 20 / 20
• Subtask 2: 30 / 30
• Subtask 3: 60 / 60