POJ 2653 Pick-up sticks（计算几何）

Pick-up sticks

Time Limit: 3000MS		Memory Limit: 65536K
Total Submissions: 7946		Accepted: 2916

Description

Stan has n sticks of various length. He throws them one at a time on the floor in a random way. After finishing throwing, Stan tries to find the top sticks, that is these sticks such that there is no stick on top of them. Stan has noticed that the last thrown stick is always on top but he wants to know all the sticks that are on top. Stan sticks are very, very thin such that their thickness can be neglected.

Input

Input consists of a number of cases. The data for each case start with 1 <= n <= 100000, the number of sticks for this case. The following n lines contain four numbers each, these numbers are the planar coordinates of the endpoints of one stick. The sticks are listed in the order in which Stan has thrown them. You may assume that there are no more than 1000 top sticks. The input is ended by the case with n=0. This case should not be processed.

Output

For each input case, print one line of output listing the top sticks in the format given in the sample. The top sticks should be listed in order in which they were thrown. 

The picture to the right below illustrates the first case from input.

Sample Input

5

1 1 4 2

2 3 3 1

1 -2.0 8 4

1 4 8 2

3 3 6 -2.0

3

0 0 1 1

1 0 2 1

2 0 3 1

0

Sample Output

Top sticks: 2, 4, 5.
Top sticks: 1, 2, 3.

Hint

Huge input,scanf is recommended.

Source

Waterloo local 2005.09.17

暴力的时候需要由前面的往后判断，这样就不会达到nlogn了

#include <iostream>

#include <cstdio>

#include <cstring>

#include <cmath>

using namespace std;

const double eps=1e-8;

int cmp(double x)

{

    if(fabs(x)<eps) return 0;

    if(x>0) return 1;

    return -1;

}

struct point

{

    double x,y;

    point(double a,double b):x(a),y(b) {}

    point() {}

};

struct line

{

    point a,b;

    line(point x,point y):a(x),b(y)  {}

    line() {}

};

double det(const point&a,const point &b)

{

    return a.x*b.y-a.y*b.x;

}

double dot(const point&a,const point &b)

{

    return a.x*b.x+a.y*b.y;

}

point operator-(const point&a,const point&b)

{

    return point(a.x-b.x,a.y-b.y);

}

point operator*(const point&a,const double &b)

{

    return point(a.x*b,a.y*b);

}

point operator*(const double &a,const point &b)

{

    return point(a*b.x,a*b.y);

}

point operator/(const point&a,const double &b)

{

    return point(a.x/b,a.y/b);

}

bool parallel(line a,line b)

{

    return !cmp(det(a.a-a.b,b.a-b.b));

}

bool line_make_point(line a,line b,point &res)

{

    if(parallel(a,b)) return false;

    double s1=det(a.a-b.a,b.b-b.a);

    double s2=det(a.b-b.a,b.b-b.a);

    res=(s1*a.b-s2*a.a)/(s1-s2);

    return true;

}

bool pointonsegment(point p,point s,point t)

{

    return cmp(det(p-s,t-s))==0&&cmp(dot(p-s,p-t))<=0;

}

line LL[100100];int vis[100100];

int main()

{

    int n;

while(scanf("%d",&n)!=EOF&&n)

{

    memset(vis,-1,sizeof(vis));

    for(int i=0;i<n;i++)

    {

        double a,b,c,d;

        scanf("%lf%lf%lf%lf",&a,&b,&c,&d);

        point A(a,b),B(c,d);

        LL.a=A; LL.b=B;

    }

    for(int i=0;i<n;i++)

    {

        for(int j=i+1;j<n;j++)

        {

            point res;

            res.x=999999;

            res.y=-99999;

            if(vis==0) break;

            if(line_make_point(LL,LL[j],res)&&pointonsegment(res,LL[j].a,LL[j].b)&&pointonsegment(res,LL.b,LL.a))

            {

                vis=0;

            }

        }

    }

    int first=1;

    printf("Top sticks: ");

    for(int i=0;i<n;i++)

    {

        if(vis==-1)

        {

            if(first==1)

            {

                first=0;

            }

            else

            {

                printf(", ");

            }

            printf("%d",i+1);

        }

    }

    printf(".\n");

}

    return 0;

}