hdu4705 Y

非连通组合个数=所有组合-连通组合个数

树形DP，根结点表示三个连通组合数的中间那个数

#include<iostream>#include<cstring>#include<cstdio>using namespace std;#define M 100010#pragma comment(linker, "/STACK:16777216")struct node{    int v,next;}edge[M*2];int head[M],tot;void insert(int u,int v){    edge[tot].v=v;    edge[tot].next=head[u];    head[u]=tot++;}int n;long long ans;int dfs(int u,int pre){    int son,tmp=0,i,v;    for(i=head[u];i!=-1;i=edge[i].next){        v=edge[i].v;        if(v==pre) continue;        son=dfs(v,u);        tmp+=son;        ans+=(long long)(n-tmp-1)*son;    }    return tmp+1;}int main(){    int u,v,i;    while(scanf("%d",&n)!=EOF){        tot=0;        ans=0;        memset(head,-1,sizeof(head));        for(i=1;i<n;i++){            scanf("%d%d",&u,&v);            insert(u,v);            insert(v,u);        }        dfs(1,-1);        long long sum=n*(n-1)*(n-2)/6;        printf("%I64d\n",sum-ans);    }    return 0;}