A Very Easy Graph Problem HDU - 6832 （最小生成树 + dfs）

  1 #include<bits/stdc++.h>
  2 #define ll long long
  3 #define MOD 1000000007 
  4 #define INF 0x3f3f3f3f3f
  5 #define mem(a,x) memset(a,x,sizeof(a))  
  6 #define _for(i,a,b) for(int i=a; i< b; i++)
  7 #define _rep(i,a,b) for(int i=a; i<=b; i++)
  8 #define ios ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
  9 
 10 using namespace std;
 11 const int MAXN = 200005 ;
 12 inline int rd() 
 13 {
 14     int res = 0,flag = 0;
 15     char ch;
 16     if ((ch = getchar()) == ‘-‘)flag = 1;
 17     else if(ch >= ‘0‘ && ch <= ‘9‘)res = ch - ‘0‘;
 18     while ((ch = getchar()) >= ‘0‘ && ch <= ‘9‘)res = (res<<1) + (res<<3) + (ch - ‘0‘);
 19     return flag ? -res : res;
 20 }
 21 //
 22 int head[MAXN]; 
 23 int num=0;
 24 struct edg{
 25    int next,to;
 26    ll w;
 27 }edge[MAXN];
 28 void edge_add(int u,int v,int w)   //链式前向星存图
 29 {
 30    num++;
 31    edge[num].next=head[u];edge[num].to=v;edge[num].w=w;head[u]=num;
 32    edge[++num].next=head[v];edge[num].to=u;edge[num].w=w;head[v]=num;
 33 }
 34 int color[MAXN];
 35 vector<int>v;
 36 struct node{
 37     int u,v;
 38     ll w;
 39 }s[MAXN];
 40 int fa[MAXN];
 41 int find(int x)
 42 {
 43      return (fa[x]==x)?x:(fa[x]=find(fa[x])); 
 44 }
 45 void kruskal(int n,int m)
 46 {
 47      int nb=0;
 48      for(int i=1;i<=m;i++){
 49          int fu=find(fa[s[i].u]),fv=find(fa[s[i].v]);
 50          if(fu!=fv){
 51             fa[fu]=fv;
 52             edge_add(s[i].u,s[i].v,s[i].w);
 53             nb++;
 54             v.push_back(s[i].w);
 55          }
 56          if(nb==n-1)break;
 57      }
 58 }
 59 ////////
 60 struct cc{
 61    int ww,bb;
 62 };
 63 int mpw[MAXN];
 64 int mpb[MAXN];
 65 ll qpow(ll a,ll b)
 66 {
 67   ll ans=1;
 68   while (b){
 69     if (b&1) ans=ans*a%MOD;
 70     a=a*a%MOD;
 71     b>>=1;
 72   }
 73   return ans;
 74 }
 75 cc dfs(int u,int fa)    //求出任意边以下的白点和黑点数
 76 {
 77    cc col;
 78    col.ww=0,col.bb=0;
 79    for(int i=head[u];i!=-1;i=edge[i].next)
 80    {
 81        int v=edge[i].to;
 82        if(v==fa)continue;
 83        cc now=dfs(v,u);
 84        col.ww+=now.ww;
 85        col.bb+=now.bb;
 86        mpw[edge[i].w]=now.ww;    //读该边以下的白点数
 87        mpb[edge[i].w]=now.bb;
 88    }
 89    cc p=col;
 90    if(color[u]==1){
 91       p.ww++;
 92       return p;
 93    }else{
 94       p.bb++;
 95       return p;
 96    }
 97 }
 98 int main()
 99 {
100     int T,n,m;
101     scanf("%d",&T);
102     while(T--){
103         num=0;
104         v.clear();
105         mem(head,-1);
106         int white=0,black=0;
107         scanf("%d %d",&n,&m);
108         for(int i=1;i<=n;i++){
109             scanf("%d",&color[i]);
110             if(color[i]==1){
111                white++;
112             }else{
113                black++;
114             }
115             mpb[i]=0;
116             mpw[i]=0;
117             fa[i]=i;
118         }
119         for(int i=1;i<=m;i++){
120             scanf("%d %d",&s[i].u,&s[i].v);
121             s[i].w=i;
122         }
123         kruskal(n,m);
124         dfs(1,0);
125         ll ans=0;
126         for(auto i:v)
127         { 
128            int nw=white-mpw[i];
129            int nb=black-mpb[i];
130            ans=(ans+(1ll*nw*mpb[i]%MOD)*(qpow(2,i)%MOD))%MOD;
131            ans=(ans+(1ll*nb*mpw[i]%MOD)*(qpow(2,i)%MOD))%MOD;
132         } 
133         printf("%lld\n",ans);
134     }
135     return 0;
136 }