【BZOJ 1007】【HNOI 2008】水平可见直线 解析几何

之前机房没网就做的这道题，用的解析几何判断交点横坐标

#include<cmath>
#include<cstdio>
#include<cstring>
#include<algorithm>
#define N 500003
#define eps 1E-8
using namespace std;
struct node {
	double k, b;
	int id;
} line[N];
int n, stack[N], top = 0;
bool ans[N];
inline bool cmp(node X, node Y) {
	if (fabs(X.k - Y.k) < eps)
		return X.b < Y.b;
	else
		return X.k < Y.k;
}
inline double crossx(int X, int Y) {
	return (line[Y].b - line[X].b) / (line[X].k - line[Y].k);
}
inline void insert(int x) {
	while (top) {
		if (fabs(line[x].k - line[stack[top]].k) < eps)
			--top;
		else
			if (crossx(x, stack[top]) <= crossx(stack[top], stack[top - 1]) && top > 1)
				--top;
			else
				break;
	}
	stack[++top] = x;
}
int main() {
	scanf("%d
", &n);
	for(int i = 1; i <= n; ++i) {
		scanf("%lf%lf
", &line[i].k, &line[i].b);
		line[i].id = i;
	}
	sort(line + 1, line + n + 1, cmp);
	for(int i = 1; i <= n; ++i)
		insert(i);
	for(int i = 1; i <= top; ++i)
		ans[line[stack[i]].id] = 1;
	for(int i = 1; i <= n; ++i)
		if (ans[i])
			printf("%d ",i);
	return 0;
}

hhh