[JSOI2008]最小生成树计数

 1 #include<iostream>
 2 #include<cstdlib>
 3 #include<cstdio>
 4 #include<cmath>
 5 #include<algorithm>
 6 #include<cstring>
 7 #include<queue>
 8 #include<vector>
 9 #define MAXN 500010
10 #define RG register
11 #define LL long long int
12 using namespace std;
13 const int INF=1e9;
14 const int mod=31011;
15 struct node{
16   int x,y,w;
17 }e[MAXN];
18 int n,m;
19 int fa[MAXN];
20 int L[MAXN],R[MAXN],T[MAXN],cnt;
21 int tot;
22 int ans=1,sum;
23 bool cmp(node a,node b){ return a.w<b.w;}
24 int find(int x)
25 {
26   if(fa[x]==x) return x;
27   else return find(fa[x]);
28 }
29 void dfs(int id,int now,int num)
30 {
31   if(now==R[id]+1)
32     {
33       if(num==T[id]) sum++;
34       return;
35     }
36   int f1=find(e[now].x),f2=find(e[now].y);
37   if(f1!=f2){
38     fa[f2]=f1;
39     dfs(id,now+1,num+1);
40     fa[f1]=f1;fa[f2]=f2;
41   }
42   dfs(id,now+1,num);
43 }
44 int main()
45 {
46   freopen("1.in","r",stdin);
47   scanf("%d%d",&n,&m);
48   for(int i=1;i<=m;i++) scanf("%d%d%d",&e[i].x,&e[i].y,&e[i].w);
49   sort(e+1,e+m+1,cmp);for(int i=1;i<=n;i++) fa[i]=i;
50   for(int i=1;i<=m;i++)
51     {
52       if(e[i].w!=e[i-1].w) R[cnt]=i-1,L[++cnt]=i;
53       int f1=find(e[i].x),f2=find(e[i].y);
54       if(f1!=f2){ fa[f2]=f1;T[cnt]++;tot++;}
55     }
56   R[cnt]=m;
57   if(tot!=n-1){ printf("0\n");return 0;}
58   for(int i=1;i<=n;i++) fa[i]=i;
59   for(int i=1;i<=cnt;i++){
60     sum=0;
61     dfs(i,L[i],0);
62     (ans*=sum)%=mod;
63     for(int j=L[i];j<=R[i];j++){
64       int f1=find(e[j].x),f2=find(e[j].y);
65       if(f1!=f2){ fa[f2]=f1;}
66     }
67   }
68   printf("%d\n",ans);
69   return 0;
70 }