[USACO07DEC]道路建设Building Roads

Posted 2022-04-14 66dzb

[Time Gate]

https://www.luogu.org/problemnew/show/P2872

【解题思路】

一道最小生成树的基本题吧，这里要注意两点

1.数组范围要开大，毕竟是生成树，开n平方即可

2.求边上权值算两点之间距离要注意精度的问题,多强制转换几次（double）防止WA

就OK了，剩下是Kruskal模板

【code】

 1 #include <cstdio>
 2 #include <cmath>
 3 #include <algorithm>
 4 using namespace std;
 5 int n,m,cnt;
 6 double ans;
 7 int x[1000005],y[1000005],fa[1000005];
 8 struct Node
 9     int x;
10     int y;
11     double dis;
12 a[1000005];
13 inline int Find(int i)
14     if(fa[i]==i)return i;
15     return fa[i]=Find(fa[i]);
16 
17 inline void Union(int x,int y)
18     int f1=Find(x);
19     int f2=Find(y);
20     if(f1!=f2)fa[f1]=f2;
21     return;
22 
23 inline bool cmp(Node a,Node b)
24     if(a.dis==b.dis)return a.x<b.x;
25     return a.dis<b.dis;
26 
27 int main()
28     scanf("%d%d",&n,&m);
29     for(register int i=1;i<=n;i++)
30         fa[i]=i;
31     for(register int i=1;i<=n;i++)
32         scanf("%d%d",&x[i],&y[i]);
33     for(register int i=1;i<=n;i++)
34         for(register int j=i+1;j<=n;j++)
35             a[++cnt].x=i;
36             a[cnt].y=j;
37             a[cnt].dis=(double)sqrt((double)(x[i]-x[j])*(double)(x[i]-x[j])+(double)(y[i]-y[j])*(double)(y[i]-y[j]));
38         
39     int x,y;
40     for(register int i=1;i<=m;i++)
41         scanf("%d%d",&x,&y);
42         a[++cnt].x=x;
43         a[cnt].y=y;
44         a[cnt].dis=0.0;
45     
46     sort(a+1,a+cnt+1,cmp);
47     for(register int i=1;i<=cnt;i++)
48         if(Find(a[i].x)!=Find(a[i].y))
49             Union(a[i].x,a[i].y);
50             ans+=a[i].dis;
51         
52     
53     printf("%.2lf\n",ans);
54     return 0;
55