图5 Saving James Bond

题目：
https://pintia.cn/problem-sets/1268384564738605056/problems/1284061680920891392
题解：https://blog.csdn.net/tuzigg123/article/details/46897871
代码：

    /*2015.7.15cyq*/
    #include <iostream>
    #include <vector>
    #include <math.h>
    #include <queue>
    #include <fstream>
    #include <map>
    using namespace std;
     
    //ifstream fin("case1.txt");
    //#define cin fin
     
    struct point{
        float x;
        float y;
        bool canEscape;
        int prePoint;//BFS时的前驱结点，便于从终点回溯到起点，用于输出路径
    };
    int main(){
        int N,D;
        cin>>N>>D;
        if(D>=50){
            cout<<"1"<<endl;
            return 0;
        }
        vector<point> vexs(N);
        vector<vector<bool> > edges(N,vector<bool>(N,false));
     
        for(int i=0;i<N;i++){//鳄鱼序号为0到N-1
            cin>>vexs[i].x>>vexs[i].y;
            if(fabs(vexs[i].x)+D>=50||fabs(vexs[i].y)+D>=50){
                vexs[i].canEscape=true;  //标记能一步跳到岸边的点
            }else
                vexs[i].canEscape=false;
        }
        for(int i=0;i<N;i++){
            for(int j=i+1;j<N;j++){
                float dx=vexs[i].x-vexs[j].x;
                float dy=vexs[i].y-vexs[j].y;
                if(dx*dx+dy*dy<=D*D){//标记能够两两连通的点
                    edges[i][j]=true;
                    edges[j][i]=true;
                }
            }
        }
        //BFS
        queue<int> cur,next;
        vector<int> visited(N,false);
        map<float,int> firstJump;
     
        for(int i=0;i<N;i++){
            float tmp=vexs[i].x*vexs[i].x+vexs[i].y*vexs[i].y;
            if(tmp<=(D+7.5)*(D+7.5)){
                firstJump[tmp]=i;  //用map对第一跳的点的距离进行自动升序排序
                vexs[i].prePoint=-1;//前驱结点为-1，表示原点
            }
        }
        for(auto it=firstJump.begin();it!=firstJump.end();++it){
                cur.push((*it).second);//第一跳能跳到的点从小到大入队
                visited[(*it).second]=true;
        }
     
        bool find=false;
        vector<int> result;
        while(!cur.empty()){
            if(find)
                break;
            while(!cur.empty()){
                int root=cur.front();
                cur.pop();
                if(vexs[root].canEscape){
                    while(vexs[root].prePoint!=-1){
                        result.push_back(root);
                        root=vexs[root].prePoint;
                    }
                    result.push_back(root);
                    find=true;
                    break;
                }
                for(int i=0;i<N;i++){
                    if(edges[root][i]&&!visited[i]){
                        next.push(i);
                        visited[i]=true;
                        vexs[i].prePoint=root;//每个结点入队时都要记录其前驱结点
                    }
                }
            }
            swap(cur,next);
        }
        
        int n=result.size();
        if(n==0)
            cout<<"0"<<endl;
        else{
            cout<<n+1<<endl;
            for(int i=n-1;i>=0;i--)
                cout<<vexs[result[i]].x<<" "<<vexs[result[i]].y<<endl;
        }
        return 0;
    }