# BZOJ-1038 [ZJOI2008]瞭望塔

#include <cstdlib>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <iostream>
#include <algorithm>
#include <cctype>
#define rep(i, l, r) for(int i=l; i<=r; i++)
#define maxn 1009
#define linf 1e15
using namespace std;
{
int x=0, f=1; char ch=getchar();
while (!isdigit(ch)) {if (ch=='-') f=-1; ch=getchar();}
while (isdigit(ch)) x=x*10+ch-'0', ch=getchar();
return x*f;
}
struct P{double x, y;} p[maxn], p2[maxn];
struct line{P a, b; double ang;} g[maxn], l[maxn], a[maxn], q[maxn];
P operator - (P a, P b){return (P){a.x-b.x, a.y-b.y};}
double operator * (P a, P b){return a.x*b.y-a.y*b.x;}
bool operator < (line a, line b){if (a.ang==b.ang) return (a.b-a.a)*(b.a-a.a)>0; return a.ang<b.ang;}
bool operator < (P a, P b){return a.x<b.x;}
inline P inter(line a, line b)
{
double k1=(b.b-a.a)*(a.b-a.a), k2=(a.b-a.a)*(b.a-a.a), t=k2/(k1+k2);
return (P){b.a.x+t*(b.b.x-b.a.x), b.a.y+t*(b.b.y-b.a.y)};
}
inline bool jud(line a, line b, line v){return (inter(a, b)-v.a)*(v.b-v.a)>0;}

int n, m, L, R, cnt;
double ans;
int main()
{
rep(i, 1, m) g[i].a=p[i], g[i].b=p[i+1];
rep(i, 1, m) l[i]=g[i], l[i].ang=atan2(l[i].b.y-l[i].a.y, l[i].b.x-l[i].a.x);
sort(l+1, l+m+1);
a[++cnt]=l[1]; rep(i, 2, m) a[a[cnt].ang!=l[i].ang ? ++cnt : cnt]=l[i];
L=1, R=0, q[++R]=a[1], q[++R]=a[2];
rep(i, 3, cnt)
{
while (L<R && jud(q[R-1], q[R], a[i])) R--;
while (L<R && jud(q[L+1], q[L], a[i])) L++;
q[++R]=a[i];
}
while (L<R && jud(q[R-1], q[R], q[L])) R--;
while (L<R && jud(q[L+1], q[L], q[R])) L++;
rep(i, L, R-1)
{
p2[i]=inter(q[i], q[i+1]);
if (p2[i].x>=p[1].x && p2[i].x<=p[m+1].x) p[++n]=p2[i];
}
sort(p+1, p+1+n);
int k1=1, k2=L; ans=linf;
rep(i, 1, n)
{
double x=p[i].x;
while (k1<m && g[k1].b.x<x) k1++;
while (k2<R && p2[k2].x<x) k2++;
line v; v.a.x=v.b.x=x, v.a.y=1, v.b.y=2;
ans=min(ans, inter(v, q[k2]).y-inter(v, g[k1]).y);
}
printf("%.3lf\n", ans);
return 0;
}
posted @ 2015-05-05 18:52  NanoApe  阅读(162)  评论(0编辑  收藏  举报