P2903 [USACO08MAR]麻煩的幹草打包機The Loathesome Hay Baler

細節題。$O(n^{2})$的$bfs$可過。

 1 #include<iostream>
 2 #include<cstdio>
 3 #include<cstdlib>
 4 #include<cstring>
 5 #include<cctype>
 6 #include<cmath>
 7 #define re register
 8 using namespace std;
 9 typedef double
 
 db;
10 void read(int &x){
11     char c=getchar();x=0;int f=1;
12     while(!isdigit(c)) f&=(c!=‘-‘),c=getchar();
13     while(isdigit(c)) x=(x<<3)+(x<<1)+(c^48),c=getchar();
14     x=f?x:-x;
15 }
16 #define N 1052
17 const db eps=1e-8;
18 int sqr(int a){return a*a;}
19 struct
 
 node{int x,y,r;}a[N];
20 int n,xt,yt,st,ed;
21 db d[N],v[N]; bool vis[N];
22 db dist(node A,node B){return sqrt((db)(sqr(A.x-B.x)+sqr(A.y-B.y)));}
23 void dfs(int x){
24     if(x==ed){
25         cout<<(int)d[x];
26         exit(0);
27     }//終點直接跳出
28     db tmp=v[x]*(db)a[x].r;
29     for
 
(int i=1;i<=n;++i){
30         if(i==x||vis[i]) continue;
31         if(fabs(dist(a[x],a[i])-(db)(a[x].r+a[i].r))<eps){//距離實時計算：兩個齒輪相切
32             vis[i]=1; v[i]=tmp/(db)a[i].r;
33             d[i]=d[x]+v[i]; dfs(i);
34         }
35     }
36 }
37 int main(){
38     read(n);read(xt);read(yt);
39     for(re int i=1;i<=n;++i){
40         read(a[i].x);read(a[i].y);read(a[i].r);
41         if(a[i].x==xt&&a[i].y==yt) ed=i;
42         if(a[i].x==0&&a[i].y==0) st=i;
43     }v[st]=d[st]=10000;vis[st]=1;dfs(st);
44 }

bzoj1615 / P2903 [USACO08MAR]麻煩的幹草打包機The Loathesome Hay Baler