bzoj2152: 聪聪可可 树分治

sb树分治

/**************************************************************
    Problem: 2152
    User: walfy
    Language: C++
    Result: Accepted
    Time:472 ms
    Memory:3720 kb
****************************************************************/
 
//#pragma comment(linker, "/stack:200000000")
//#pragma GCC optimize("Ofast,no-stack-protector")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
//#pragma GCC optimize("unroll-loops")
#include<bits/stdc++.h>
#define fi first
#define se second
#define mp make_pair
#define pb push_back
#define pi acos(-1.0)
#define ll long long
#define vi vector<int>
#define mod 1000000007
#define C 0.5772156649
#define ls l,m,rt<<1
#define rs m+1,r,rt<<1|1
#define pil pair<int,ll>
#define pli pair<ll,int>
#define pii pair<int,int>
#define cd complex<double>
#define ull unsigned long long
#define base 1000000000000000000
#define fio ios::sync_with_stdio(false);cin.tie(0)
 
using namespace std;
 
const double eps=1e-6;
const int N=20000+10,maxn=5000+10,inf=0x3f3f3f3f,INF=0x3f3f3f3f3f3f3f3f;
 
struct edge{
    int to,Next,c;
}e[N*4];
int cnt,head[N],num[10];
void init()
{
    cnt=0;
    memset(head,-1,sizeof head);
}
void add(int u,int v,int c)
{
    e[cnt].to=v;
    e[cnt].c=c;
    e[cnt].Next=head[u];
    head[u]=cnt++;
}
int vis[N],sz[N],zx[N];
void dfssz(int u,int f)
{
    sz[u]=1;
    for(int i=head[u];~i;i=e[i].Next)
    {
        int x=e[i].to;
        if(x==f||vis[x])continue;
        dfssz(x,u);
        sz[u]+=sz[x];
    }
}
void dfszx(int u,int f,int o,int &ans)
{
    zx[u]=sz[o]-sz[u];
    for(int i=head[u];~i;i=e[i].Next)
    {
        int x=e[i].to;
        if(x==f||vis[x])continue;
        zx[u]=max(zx[u],sz[x]);
        dfszx(x,u,o,ans);
    }
    if(ans==-1||zx[ans]>zx[u])ans=u;
}
int findzx(int u,int f)
{
    dfssz(u,f);
    int ans=-1;
    dfszx(u,f,u,ans);
    return ans;
}
void dfsroot(int u,int f,int sum)
{
    num[sum%3]++;
    for(int i=head[u];~i;i=e[i].Next)
    {
        int x=e[i].to;
        if(x==f||vis[x])continue;
        dfsroot(x,u,sum+e[i].c);
    }
}
int cal()
{
    return num[0]*(num[0]-1)/2+num[1]*num[2];
}
int solve(int o,int f)
{
    int root=findzx(o,f);
    memset(num,0,sizeof num);
    dfsroot(root,-1,0);
//    printf("++++%d %d %d %d\n",num[0],num[1],num[2],cal());
    int ans=cal();
    vis[root]=1;
    for(int i=head[root];~i;i=e[i].Next)
    {
        int x=e[i].to;
        if(x==f||vis[x])continue;
        memset(num,0,sizeof num);
        dfsroot(x,root,e[i].c);
//        printf("----%d %d %d %d\n",num[0],num[1],num[2],cal());
        ans-=cal();
    }
    for(int i=head[root];~i;i=e[i].Next)
    {
        int x=e[i].to;
        if(x==f||vis[x])continue;
        ans+=solve(x,root);
    }
    return ans;
}
int main()
{
    int n;
    scanf("%d",&n);
    init();
    for(int i=1;i<n;i++)
    {
        int a,b,c;
        scanf("%d%d%d",&a,&b,&c);
        add(a,b,c);add(b,a,c);
    }
    int ans1=solve(1,-1)*2+n;
    int ans2=n*n;
    int p=__gcd(ans1,ans2);
    printf("%d/%d\n",ans1/p,ans2/p);
    return 0;
}
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