文章标题 HDU 5977 : Garden of Eden （树分治）

参考自：http://blog.youkuaiyun.com/bahuia/article/details/53070036
代码：

#include <iostream>
#include <cstdio>
#include <cstring>
#include <string>
#include <queue>
#include <set>
#include <map>
#include <algorithm>
#include <math.h>
#include <vector>
using namespace std;
typedef long long ll;

const int inf = 0x3f3f3f3f;
const int mod=1e9+7;
const int maxn=5e4+10;

int n,k,a[maxn];
vector<int>sta;
ll ha[1100],ans;

struct Edge{
    int to,nex;
}edge[maxn*2];
int tot,head[maxn],vis[maxn];
void init(){
    tot=0;
    memset (head,-1,sizeof (head));
    memset (vis,0,sizeof (vis));
}
void addedge(int u,int v){
    edge[tot]=Edge{v,head[u]};
    head[u]=tot++; 
}

int sz[maxn],maxv[maxn],rt,Max,root;

void dfs_size(int u,int pre){//求出每个子树的大小，以及每个节点的最大儿子 
    sz[u]=1;
    maxv[u]=0;
    for (int i=head[u];i!=-1;i=edge[i].nex){
        int v=edge[i].to;
        if (v==pre||vis[v])continue;
        dfs_size(v,u);
        sz[u]+=sz[v];
        maxv[u]=max(maxv[u],sz[v]);
    }
}

void dfs_root(int r,int u,int pre){//找出以u为根的子树的重心
    maxv[u]=max(maxv[u],sz[r]-maxv[u]);
    if (Max>maxv[u]){
        Max=maxv[u];root=u;
    }
    for (int i=head[u];i!=-1;i=edge[i].nex){
        int v=edge[i].to;
        if (v==pre||vis[v])continue;
        dfs_root(r,v,u);
    }
}

void dfs_sta(int u,int fa,int s){
    sta.push_back(s);
    for (int i=head[u];i!=-1;i=edge[i].nex){
        int v=edge[i].to;
        if (v==fa||vis[v])continue;
        dfs_sta(v,u,s|(1<<a[v]));
    }
}

ll solve(int u,int s){//计算当前子树中合法的点对数
    ll res=0;
    sta.clear();
    dfs_sta(u,-1,s);
    memset (ha,0,sizeof (ha));
    for (int i=0;i<sta.size();i++)ha[sta[i]]++;
    for (int i=0;i<sta.size();i++){
        ha[sta[i]]--;
        res+=ha[(1<<k)-1];
        for (int s0=sta[i];s0;s0=(s0-1)&sta[i]){
            res+=ha[((1<<k)-1)^s0]; 
        }
        ha[sta[i]]++;
    }
    return res;
} 

void dfs(int u,int num){//总的dfs求解
    Max=n;
    dfs_size(u,-1);
    dfs_root(u,u,-1);
    int rt=root;//一定要注意这样里的root是全局变量，在递归之后可能改变，需要提前保存下来。
    vis[rt]=1;
    ans+=solve(rt,1<<(a[rt]));
    for (int i=head[rt];i!=-1;i=edge[i].nex){
        int v=edge[i].to;
        if (vis[v])continue;
        ans-=solve(v,(1<<a[v])|(1<<a[rt]));
    }
    for (int i=head[rt];i!=-1;i=edge[i].nex){
        int v=edge[i].to;
        if (vis[v])continue;
        dfs(v,sz[v]);
    }
}



int main()
{
    while (scanf ("%d%d",&n,&k)!=EOF){
        init();
        for (int i=1;i<=n;i++){
            scanf ("%d",&a[i]);
            a[i]--;
        }
        int u,v;
        for (int i=0;i<n-1;i++){
            scanf ("%d%d",&u,&v);
            addedge(u,v);
            addedge(v,u); 
        }
        if (k==1){
            printf ("%lld\n",(ll)n*n);
            continue;
        }
        ans=0;
        dfs(1,n);
        printf ("%lld\n",ans);
    }
    return 0;
}
/*
3 2
1 2 2
1 2
1 3
*/