UVALive 4992 Jungle Outpost(半平面交判存)

#include <iostream>
#include <cstdio>
#include <cstring>
#include <string>
#include <cmath>
#include <vector>
#include <queue>
#include <map>
#include <set>
#include <stack>
#include <algorithm>

using namespace std;

#define X first
#define Y second
#define root 1,n,1
#define lr rt<<1
#define rr rt<<1|1
#define lson l,mid,rt<<1
#define rson mid+1,r,rt<<1|1

typedef long long LL;
typedef pair<int,int> pii;
const int N = 50050 ;
const int inf = 1e9+7;
const double eps = 1e-8;
int n ;

inline int dcmp(double x){
    if( fabs(x)<eps ) return 0;
    return x<0?-1:1;
}
struct Point {
    double x , y ;
    Point(){};
    Point(double x,double y):x(x),y(y){};
    bool operator < ( const Point &a ) const {
        return dcmp(x-a.x) < 0 || ( dcmp(x-a.x)==0 && dcmp(y-a.y) < 0 );
    }
}e[N];
typedef Point Vector;
struct Line {
    Point p;
    Vector v ;          // directional vector
    double ang ;
    Line(){};
    Line( Point p , Vector v ):p(p),v(v){ ang = atan2(v.y,v.x); }
    bool operator < ( const Line &L ) const {
        return ang < L.ang;
    }
};

Point operator - ( Point a , Point b ){ return Point(a.x-b.x,a.y-b.y); }
Point operator + ( Point a , Point b ){ return Point(a.x+b.x,a.y+b.y); }
Point operator * ( Point a , double p ){ return Point(a.x*p,a.y*p); }

inline double Cross(Point a , Point b ) { return a.x*b.y-a.y*b.x;}
bool OnLeft( Line L , Point p ) {
    return Cross( L.v , p-L.p ) > 0 ;
}

Point GetIntersection( Line a , Line b ){
    Vector u = a.p - b.p ;
    double t = Cross(b.v,u) / Cross(a.v,b.v);
    return a.p + a.v*t;
}

bool HPI( Line* L , int n  ) {
    sort( L , L + n );
    int first , last ;          //deque ,
    Point *p = new Point[n];    //p[i] is q[i] and q[i+1] ‘s intersection point
    Line *q = new Line[n];
    q[first=last=0] = L[0];
    for( int i = 1 ; i < n ; ++i ) {
        while( first < last && !OnLeft( L[i], p[last-1] )) last--;
        while( first < last && !OnLeft( L[i], p[first] )) first++;
        q[++last] = L[i];
        if( fabs(Cross(q[last].v,q[last-1].v)) < eps ) {
            last--;
            if( OnLeft( q[last] , L[i].p) ) q[last] = L[i];
        }
        if( first < last ) p[last-1] = GetIntersection(q[last-1],q[last]);
    }
    while( first < last && !OnLeft( q[first],p[last-1])) last--;
    // delete useless plane
    if( last - first <= 1 ) return false ;
    return true ;
}

Line L[N];

bool ok( int num ) {
    int tot = 0 ;
    for( int i = 0 ; i < n ; ++i ) {
        L[tot++] = Line( e[(i+num+1)%n],e[i]-e[(i+num+1)%n] );   // convex in clockwise direction
    }
    if( !HPI(L,tot) ) return true ;
    return false ;
}

void Run() {

    for( int i = 0 ; i < n ; ++i ) {
        scanf("%lf%lf",&e[i].x,&e[i].y);
    }
    if( n == 3 ) { puts("1"); return ; }
    int ans = 0 , l = 0 , r = n - 3 ;
    while( l <= r ) {
        int mid = (l+r)>>1;
        if( ok(mid) )
            ans = mid , r = mid - 1 ;
        else
            l = mid + 1 ;
    }
    printf("%d\n",ans);
}

int main(){
    #ifdef LOCAL
        freopen("in.txt","r",stdin);
    #endif // LOCAL
    while( scanf("%d",&n)!=EOF ) Run();
}