[ZJOI2012]旅游 对偶图 树的直径

Code:

// luogu-judger-enable-o2
#include<cstdio>
#include<iostream>
#include<algorithm>
#include<map>
#include<vector>
#include<string>
#include<cmath>
using namespace std;
void setIO(string a){
    freopen((a+".in").c_str(),"r",stdin);
    freopen((a+".out").c_str(),"w",stdout);
}
void shutIO(){
    fclose(stdin),fclose(stdout);
}
#define O2 __attribute__((optimize("-O3")))
#define maxn 300000
#define P pair<int,int>
map<P,int>key;
vector<int>G[maxn];
int idx,nodes,ans;
O2 void insert(int a,int b,int i){
    if(!key[P(a,b)]) key[P(a,b)]=key[P(b,a)]=++idx;
    int cur=key[P(a,b)];
    G[cur].push_back(i);
}
int head[maxn], to[maxn<<1],nex[maxn<<1],cnt, d1[maxn],d2[maxn];
O2 void addedge(int u,int v){
    nex[++cnt]=head[u],head[u]=cnt,to[cnt]=v;
}
O2 void dfs(int u,int fa){
    d1[u]=d2[u]=0;
    for(int v=head[u];v;v=nex[v]){
        if(to[v]==fa) continue;
        dfs(to[v],u);
        if(d1[to[v]]>d1[u]) d2[u]=d1[u],d1[u]=d1[to[v]];
        else if(d1[to[v]]>d2[u]) d2[u]=d1[to[v]];
    }
    ans=max(ans,d1[u]+d2[u]+1);
    ++d1[u],++d2[u];
}
O2 int main(){
    //setIO("shop");
    int n;
    scanf("%d",&n);
    for(nodes=1;nodes<=n-2;++nodes){
        int a,b,c;
        scanf("%d%d%d",&a,&b,&c);
        if(abs(a-b)!=1)insert(a,b,nodes);
        if(abs(a-c)!=1)insert(a,c,nodes);
        if(abs(b-c)!=1)insert(b,c,nodes);
    }
    for(int i=1;i<=idx;++i){
    	int siz=G[i].size()-1;
        for(int j=0;j<siz;++j)
            addedge(G[i][j],G[i][j+1]),addedge(G[i][j+1],G[i][j]);
    }
    dfs(1,0);
    printf("%d",ans);
    //shutIO();
    return 0;
}