手動博客搬家: 本文發(fā)表于20181208 14:39:01, 原地址https://blog.csdn.net/suncongbo/article/details/84891710

哇它居然顯示出圖片來了……感動啊

題目鏈接:https://www.lydsy.com/JudgeOnline/problem.php?id=4739http://uoj.ac/problem/277

不難得出一個結(jié)論: 兩圓之間最短路一定沿著圓的公切線走。然后得到如下算法：每兩個圓之間作公切線(4條)，如果一條公切線不穿過其他圓，就把這兩個點(圖論中的點)之間連上邊，邊權(quán)為切線長；同一圓上相鄰兩點連邊，邊權(quán)為圓上距離，然后S,T分別向每個圓作切線如果不和其他圓相交就連邊。這樣的話點數(shù)、邊數(shù)是\(O(n^2)\)級別的，使用Dijkstra算法求最短路即可。時間復(fù)雜度瓶頸在于對于一條邊枚舉每一個圓去check是否相交，總時間復(fù)雜度\(O(n^3)\). 奇跡般地能跑過。
下面重點講解一下我是如何求兩圓公切線以及點和圓的切線的:
先來說兩圓公切線：因為兩圓外離所以一定有\(4\)條公切線，\(2\)內(nèi)\(2\)外。
考慮先把圓按半徑從大到小排序，現(xiàn)在要從半徑大的圓向半徑小的圓作\(4\)條切線。
設(shè)兩圓圓心(已知)分別為\(O_1, O_2\), 圓心的直線距離\(O_1O_2=d\), 半徑分別為\(r_1r_2\), 一條外公切線為\(AB\)(未知)。
過\(O_2\)作\(O_2E\perp AO_1\)于\(D\), 則\(AEO_2B\)為矩形。\(AE=BO_2=r_2\), \(EO_1=AO_1-AE=r_1-r_2\), 又\(O_1E\perp O_2E\), 可用勾股定理算出切線長\(AB=EO_2=\sqrt{d^2-(r_1-r_2)^2}\). 同時有\(\cos \angle EO_1O_2=\frac{r_1-r_2}ze8trgl8bvbq\), 可利用acos函數(shù)算出\(\angle EO_1O_2\)的值。然后求出向量\(\vec {OA}=\frac{r_1}ze8trgl8bvbq\vec v\)即可，其中\(\vec v\)為\(\vec {O_1O_2}\)逆時針旋轉(zhuǎn)\(\angle EO_1O_2\)的向量。

另外一條外公切線同理，把旋轉(zhuǎn)度數(shù)取反即可。
代碼如下: Line tmp; double d = EuclidDist(a[i].o,a[j].o); Vector v; double ang; Point p1,p2;//Out 1ang = acos((a[i].r-a[j].r)/d); v = rotate(Vector(a[j].o-a[i].o),ang)/d;p1 = a[i].o+v*a[i].r,p2 = a[j].o+v*a[j].r;tmp = Line(p1,p2);//Out 2v = rotate(Vector(a[j].o-a[i].o),-ang)/d;p1 = a[i].o+v*a[i].r,p2 = a[j].o+v*a[j].r;tmp = Line(p1,p2);

對于兩條內(nèi)公切線，只要\(EO_2=\sqrt{d^2-(r_1+r_2)^2}\)然后計算即可。注意內(nèi)公切線的向量是一加一減。

//In 1ang = acos((a[i].r+a[j].r)/d); v = rotate(Vector(a[j].o-a[i].o),ang)/d;p1 = a[i].o+v*a[i].r,p2 = a[j].o-v*a[j].r;tmp = Line(p1,p2);//In 2v = rotate(Vector(a[j].o-a[i].o),-ang)/d;p1 = a[i].o+v*a[i].r,p2 = a[j].o-v*a[j].r;tmp = Line(p1,p2);

然后再來看過一點作圓的兩條切線:
和剛才的做法類似，逆時針旋轉(zhuǎn)向量\(\vec {OP}\) \(\arcsin \frac{r}ze8trgl8bvbq\)角即可。然后再除以\(d\)乘以\(\sqrt{d^2-r^2}\).

代碼如下: Line tmp; double d; Vector v; Point p; double ang;//1d = EuclidDist(s1,a[i].o); ang = asin(a[i].r/d); v = rotate(Vector(a[i].o-s1),ang)/d;p = s1+v*sqrt(d*d-a[i].r*a[i].r);tmp = Line(s1,p);//2v = rotate(Vector(a[i].o-s1),-ang)/d;p = s1+v*sqrt(d*d-a[i].r*a[i].r);tmp = Line(s1,p);

另外的一些計算幾何問題:

如何check: 對于一個圓判斷當(dāng)前線段到這個圓心的距離是否大于半徑。點到線段的距離: 利用點積判斷是否在端點兩側(cè)(和端點的夾角是否為鈍角)，如果不是利用叉積求點到直線的距離即可。

il double PointDisttoSegment(Line l,Point x) {Vector v1 = l.y-l.x,v2 = x-l.x,v3 = x-l.y;if(dcmp(Dot(v1,v2))<0) return EuclidDist(x,l.x);else if(dcmp(Dot(v1,v3))>0) return EuclidDist(x,l.y);else return fabs(Cross(v2,v3))/EuclidDist(l.x,l.y); }

如何求圓上距離
設(shè)圓半徑為\(r\), 兩點\(AB\)距離為\(d\), 則圓心角為\(\theta=2\arcsin{\fracze8trgl8bvbq{2r}}\).或者等于\(\arctan k_1-\arctan k_2\), \(k_1, k_2\)分別是\(OA, OB\)的斜率。兩種計算方法均可，精度哪個更好未知。我使用了第二種。然后圓上距離就等于\(r\theta\). (當(dāng)時寫成了\(2\pi r \theta\)調(diào)了好長時間) 注意特判負(fù)角度，不要走劣弧。

代碼實現(xiàn)

7kb多一點……

#include<cstdio> #include<cstdlib> #include<cstring> #include<cmath> #include<vector> #include<algorithm> #include<map> #include<queue> #define llong long long #define il inline #define modinc(x) {if(x>=P) x-=P;} using namespace std;const int N1 = 502; const double eps = 1e-15; const int N = 2e6+2e3+2; const int M = 4.25e6; const double PI = 3.1415926535898; const double INF = 1e9; struct Point {double x,y;Point() {}Point(double _x,double _y) {x = _x,y = _y;}double length() {return sqrt(x*x+y*y);}bool operator ==(const Point &arg) const{return fabs(x-arg.x)<eps && fabs(y-arg.y)<eps;}bool operator <(const Point &arg) const{return x<arg.x || (x==arg.x && y<arg.y);} }; typedef Point Vector; Point operator +(Point x,Vector y) {return Point(x.x+y.x,x.y+y.y);} Point operator -(Point x,Vector y) {return Point(x.x-y.x,x.y-y.y);} Vector operator *(Vector x,double y) {return Vector(x.x*y,x.y*y);} Vector operator /(Vector x,double y) {return Vector(x.x/y,x.y/y);} il double Cross(Vector x,Vector y) {return x.x*y.y-x.y*y.x;} il double Dot(Vector x,Vector y) {return x.x*y.x+x.y*y.y;} il int dcmp(double x) {return x<-eps ? -1 : (x>eps ? 1 : 0);} il double EuclidDist(Point x,Point y) {return sqrt((x.x-y.x)*(x.x-y.x)+(x.y-y.y)*(x.y-y.y));} il Vector rotate(Vector x,double ang) {return Point(x.x*cos(ang)-x.y*sin(ang),x.x*sin(ang)+x.y*cos(ang));} struct Circle {Point o; double r;Circle() {}Circle(Point _o,double _r) {o = _o,r = _r;}bool operator <(const Circle &arg) const{return r<arg.r || (r==arg.r && o<arg.o);} }; struct Line {Point x,y;Line() {}Line(Point _x,Point _y) {x = _x,y = _y;} }; struct Element {Line l; int id1,id2;Element() {}Element(Line _l,int _id1,int _id2) {l = _l,id1 = _id1,id2 = _id2;} } q[M+2]; struct Edge {int v,nxt; double w; } e[(M<<1)+2]; struct DijNode {int id; double dis;DijNode() {}DijNode(int _id,double _dis) {id = _id,dis = _dis;}bool operator <(const DijNode &x) const{return dis>x.dis;} }; int fe[N+3]; double dis[N+3]; int vis[N+3]; priority_queue<DijNode> pq; Circle a[N+3]; vector<Point> disc; map<Point,int> mp; vector<Point> belong[N1+3]; Point s1,t1,curo; int n,tp,en;il double PointDisttoSegment(Line l,Point x) {Vector v1 = l.y-l.x,v2 = x-l.x,v3 = x-l.y;if(dcmp(Dot(v1,v2))<0) return EuclidDist(x,l.x);else if(dcmp(Dot(v1,v3))>0) return EuclidDist(x,l.y);else return fabs(Cross(v2,v3))/EuclidDist(l.x,l.y); }bool check(Line l,int x,int y) {for(int i=1; i<=n; i++){if(i==x || i==y) continue;double tmp = PointDisttoSegment(l,a[i].o);if(dcmp(tmp-a[i].r)==-1) return false;}return true; }il void addedge(int u,int v,double w) {en++; e[en].v = v; e[en].w = w;e[en].nxt = fe[u]; fe[u] = en; }il bool cmp(Circle x,Circle y) {return y<x;} il bool cmp1(Point x,Point y) {Vector xx = x-curo,yy = y-curo;return dcmp(atan2(xx.y,xx.x)-atan2(yy.y,yy.x))<0; } int getid(Point x) {return lower_bound(disc.begin(),disc.end(),x)-disc.begin()+1;}double Dijkstra(int s,int t) {for(int i=1; i<=disc.size(); i++) dis[i] = INF;pq.push(DijNode(s,0.0)); dis[s] = 0.0;while(!pq.empty()){DijNode tmp = pq.top(); pq.pop(); int u = tmp.id;if(tmp.dis!=dis[u]) continue;if(vis[u]) continue;vis[u] = true;for(int i=fe[u]; i; i=e[i].nxt){if(vis[e[i].v]==false && dcmp(dis[e[i].v]-dis[u]-e[i].w)==1){dis[e[i].v] = dis[u]+e[i].w;pq.push(DijNode(e[i].v,dis[e[i].v]));}}}return dis[t]; }int main() {scanf("%lf%lf%lf%lf",&s1.x,&s1.y,&t1.x,&t1.y);scanf("%d",&n);for(int i=1; i<=n; i++){scanf("%lf%lf%lf",&a[i].o.x,&a[i].o.y,&a[i].r);}sort(a+1,a+n+1,cmp);tp = 0;for(int i=1; i<=n; i++){for(int j=i+1; j<=n; j++){Line tmp; double d = EuclidDist(a[i].o,a[j].o); Vector v; double ang; Point p1,p2;ang = acos((a[i].r-a[j].r)/d); v = rotate(Vector(a[j].o-a[i].o),ang)/d;p1 = a[i].o+v*a[i].r,p2 = a[j].o+v*a[j].r;tmp = Line(p1,p2);bool ok = check(tmp,i,j);if(ok) {tp++; q[tp] = Element(tmp,i,j); belong[i].push_back(p1); belong[j].push_back(p2);}v = rotate(Vector(a[j].o-a[i].o),-ang)/d;p1 = a[i].o+v*a[i].r,p2 = a[j].o+v*a[j].r;tmp = Line(p1,p2);ok = check(tmp,i,j);if(ok) {tp++; q[tp] = Element(tmp,i,j); belong[i].push_back(p1); belong[j].push_back(p2);}ang = acos((a[i].r+a[j].r)/d); v = rotate(Vector(a[j].o-a[i].o),ang)/d;p1 = a[i].o+v*a[i].r,p2 = a[j].o-v*a[j].r;tmp = Line(p1,p2);ok = check(tmp,i,j);if(ok) {tp++; q[tp] = Element(tmp,i,j); belong[i].push_back(p1); belong[j].push_back(p2);}v = rotate(Vector(a[j].o-a[i].o),-ang)/d;p1 = a[i].o+v*a[i].r,p2 = a[j].o-v*a[j].r;tmp = Line(p1,p2);ok = check(tmp,i,j);if(ok) {tp++; q[tp] = Element(tmp,i,j); belong[i].push_back(p1); belong[j].push_back(p2);}}}for(int i=1; i<=n; i++){Line tmp; double d; Vector v; Point p; double ang;d = EuclidDist(s1,a[i].o); ang = asin(a[i].r/d); v = rotate(Vector(a[i].o-s1),ang)/d;p = s1+v*sqrt(d*d-a[i].r*a[i].r);tmp = Line(s1,p);bool ok = check(tmp,0,i);if(ok) {tp++; q[tp] = Element(tmp,0,i); belong[i].push_back(p);}v = rotate(Vector(a[i].o-s1),-ang)/d;p = s1+v*sqrt(d*d-a[i].r*a[i].r);tmp = Line(s1,p);ok = check(tmp,0,i);if(ok) {tp++; q[tp] = Element(tmp,0,i); belong[i].push_back(p);}d = EuclidDist(t1,a[i].o); ang = asin(a[i].r/d); v = rotate(Vector(a[i].o-t1),ang)/d;p = t1+v*sqrt(d*d-a[i].r*a[i].r);tmp = Line(p,t1);ok = check(tmp,i,0);if(ok) {tp++; q[tp] = Element(tmp,i,0); belong[i].push_back(p);}v = rotate(Vector(a[i].o-t1),-ang)/d;p = t1+v*sqrt(d*d-a[i].r*a[i].r);tmp = Line(p,t1);ok = check(tmp,i,0);if(ok) {tp++; q[tp] = Element(tmp,i,0); belong[i].push_back(p);}}for(int i=1; i<=tp; i++){disc.push_back(q[i].l.x); disc.push_back(q[i].l.y);}disc.push_back(s1); disc.push_back(t1);sort(disc.begin(),disc.end());disc.erase(unique(disc.begin(),disc.end()),disc.end());for(int i=0; i<disc.size(); i++){mp[disc[i]] = i+1;}for(int i=1; i<=n; i++){curo = a[i].o;sort(belong[i].begin(),belong[i].end(),cmp1);belong[i].erase(unique(belong[i].begin(),belong[i].end()),belong[i].end());if(belong[i].size()==1) continue;for(int j=0; j<belong[i].size(); j++){int prv = j-1; if(prv==-1) prv = belong[i].size()-1;Vector v1 = belong[i][j]-a[i].o,v2 = belong[i][prv]-a[i].o;double ang = fabs(atan2(v1.y,v1.x)-atan2(v2.y,v2.x));if(dcmp(ang-PI)==1) ang = 2.0*PI-ang;addedge(mp[belong[i][j]],mp[belong[i][prv]],ang*a[i].r);addedge(mp[belong[i][prv]],mp[belong[i][j]],ang*a[i].r);}}for(int i=1; i<=tp; i++){addedge(mp[q[i].l.x],mp[q[i].l.y],EuclidDist(q[i].l.x,q[i].l.y));addedge(mp[q[i].l.y],mp[q[i].l.x],EuclidDist(q[i].l.x,q[i].l.y));}if(check(Line(s1,t1),0,0)){addedge(mp[s1],mp[t1],EuclidDist(s1,t1));addedge(mp[t1],mp[s1],EuclidDist(s1,t1));}double ans = Dijkstra(mp[s1],mp[t1]);printf("%.1lf\n",ans);return 0; }

總結(jié)

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