poj1328

　　觉得自己的做法和网上的有点不同，所以发一下。首先这道题是用贪心算法的，但是我采用的判定值和网上的不太一样。我的思路是从左往右放雷达，采用了贪心选择值x（圆心的坐标），按这个值对岛屿排序。拍完序后从左边开始放雷达，放在岛屿队列中第一个岛屿的x点处，如果雷达能监控到岛屿序列中的下一个岛屿a，则跳过岛屿a，否则再为a放一个雷达，依此类推。

代码如下

#include <iostream>

using namespace std;

#include <math.h>



struct island

{

    int x;

    int y;

};



void quick_sort(island *islands,int start,int end,int d);

int partition(island *islands,int start,int end,int d);

bool smaller_than(island *is1,island *is2,int d);



void sort(island *islands,int n,int d)

{

    quick_sort(islands,0,n-1,d);

}



void quick_sort(island *islands,int start,int end,int d)

{

    if(start>=end) return;

    int mid=partition(islands,start,end,d);

    quick_sort(islands,start,mid-1,d);

    quick_sort(islands,mid+1,end,d);

}



int partition(island *islands,int start,int end,int d)

{

    int front,back,dir=0;

    island tis=islands[start];

    

    front=start;

    back=end;

    while(front<back)

    {

        if(dir==0)

        {

            if(!smaller_than(&tis,islands+back,d)){

                islands[front].x=islands[back].x;

                islands[front].y=islands[back].y;

                front++;

                dir=1;

            }

            else back--;

        }

        else 

        {

            if(smaller_than(&tis,islands+front,d)){

                islands[back].x=islands[front].x;

                islands[back].y=islands[front].y;

                back--;

                dir=0;

            }

            else front++;

        }

    }

    islands[front].x=tis.x;

    islands[front].y=tis.y;

    return front;

}



bool smaller_than(island *is1,island *is2,int d)

{

    double v1=is1->x+sqrt(double(d*d-is1->y*is1->y)),v2=is2->x+sqrt(double(d*d-is2->y*is2->y));

    return v1<v2;

}





int place_radar(island *islands,int n,int d)

{

    int i,counter=0;

    double rx,ry=0;

    i=0;

    while(i<n)

    {

        rx=islands[i].x+sqrt(double(d*d-islands[i].y*islands[i].y));

        counter++;

        i++;

        if(i>=n) break;

        while((rx-islands[i].x)*(rx-islands[i].x)+(ry-islands[i].y)*(ry-islands[i].y)<=d*d) 

        {

            i++;

            if(i>=n) break;

        }

    }

    return counter;

}



int main()

{

    //freopen("data.txt","r",stdin);



    int i,n,d,counter=1,result;



    cin>>n>>d;

    while(n!=0||d!=0)

    {

        island *islands=new island[n];

        for(i=0;i<n;i++) cin>>islands[i].x>>islands[i].y;



        bool flag=true;

        for(i=0;i<n;i++) if(islands[i].y>d) {flag=false; break;}

        if(d<0) flag=false;



        if(!flag) result =-1;

        else{

            sort(islands,n,d);

            result=place_radar(islands,n,d);        

        }



        cout<<"Case "<<counter<<": "<<result<<endl;

        delete []islands;

        counter++;

        cin>>n>>d;

    }





    return 0;

}

另一篇文章及测试数据：http://blog.csdn.net/qqhuxiao/article/details/7686568