cf734 E. Anton and Tree

这个题的题意还是很劲的。搞了好久才知道是怎么变得。

（假设已经缩好了点，每次边中间颜色不同的，然后和就和他外面的相同，继续再变这个大的，依次类推，最多就是树的直径D/2）

（还是英语水平太弱了（吐槽++，数学考得还没英语高，还是找个地方撞死吧2333））

 #include<bits/stdc++.h>
 #define lowbit(x) x&(-x)
 #define LL long long
 #define N 200005
 #define M 1000005
 #define mod 1000000007LL
 #define inf 0x7ffffffff
 using namespace std;
 inline int ra()
 {
     int x=,f=; char ch=getchar();
     while (ch<'' || ch>''){if (ch=='-') f=-; ch=getchar();}
     while (ch>='' && ch<=''){x=x*+ch-''; ch=getchar();}
     return x*f;
 }
 struct node{
     int next,to;
 }re[N<<],e[N<<];
 int head[N],hd[N],cnt,recnt;
 void insert(int x, int y)
 {
     e[++cnt].to=y;
     e[cnt].next=head[x];
     head[x]=cnt;
 }
 void reinsert(int x, int y)
 {
     re[++recnt].to=y;
     re[recnt].next=hd[x];
     hd[x]=recnt;
 }
 int ans,sum[N],belong[N],mx[N],sec[N],n,deep[N];
 bool col[N];
 void dfs(int x, int num)
 {
     belong[x]=num;
     for (int i=head[x];i;i=e[i].next)
     {
         if (belong[e[i].to]) continue;
         if (col[e[i].to]==col[x]) dfs(e[i].to,num);
     }
 }
 void rebuild()
 {
     for (int i=; i<=n; i++)
     {
         for (int j=head[i];j;j=e[j].next)
         {
             if (belong[e[j].to]==belong[i]) continue;
             reinsert(belong[i],belong[e[j].to]);
         }
     }
 }
 void dfs1(int x, int fa)
 {
     deep[x]=; int mx=,sc=;
     for (int i=hd[x];i;i=re[i].next)
     {
         if (re[i].to==fa) continue;
         dfs1(re[i].to,x);
         deep[x]=max(deep[x],deep[re[i].to]+);
         if (mx==deep[re[i].to]) sc=mx;
         if (mx<deep[re[i].to]) sc=mx,mx=deep[re[i].to];
         if (mx>deep[re[i].to]) sc=max(sc,deep[re[i].to]);
     }
     ans=max(ans,max(deep[x],mx+sc+));
 }
 int main()
 {
     n=ra();
     for (int i=; i<=n; i++) col[i]=ra();
     for (int i=; i<n; i++)
     {
         int x=ra(),y=ra();
         insert(x,y);
         insert(y,x);
     }
     int tot=;
     for (int i=; i<=n; i++)
         if (!belong[i]) dfs(i,++tot);
     rebuild();
     dfs1(,);
     cout<<ans/;
     return ;
 } //一开始没看懂题意2333，以为好水
 /*#include<bits/stdc++.h>
 #define lowbit(x) x&(-x)
 #define LL long long
 #define N 200005
 #define M 1000005
 #define mod 1000000007LL
 #define inf 0x7ffffffff
 using namespace std;
 inline int ra()
 {
     int x=0,f=1; char ch=getchar();
     while (ch<'0' || ch>'9'){if (ch=='-') f=-1; ch=getchar();}
     while (ch>='0' && ch<='9'){x=x*10+ch-'0'; ch=getchar();}
     return x*f;
 }
 struct node{
     int next,to;
 }e[N<<1];
 int head[N],cnt;
 bool vis[N],col[N];
 inline void insert(int x, int y)
 {
     e[++cnt].to=y;
     e[cnt].next=head[x];
     head[x]=cnt;
 }
 void dfs(int x, int fa)
 {
     vis[x]=1;
     for (int i=head[x];i;i=e[i].next)
     {
         if (e[i].to==fa || col[e[i].to]!=col[x] || vis[e[i].to]) continue;
         dfs(e[i].to,x);
     }
 }
 int n,ans1,ans0;
 int main()
 {
     n=ra();
     for (int i=1; i<=n; i++) col[i]=ra();
     for (int i=1; i<n; i++)
     {
         int x=ra(),y=ra();
         insert(x,y);
         insert(y,x);
     }
     for (int i=1; i<=n; i++)
         if (col[i]==0 && !vis[i]) ++ans0,dfs(i,0);
     memset(vis,0,sizeof(vis));
     for (int i=1; i<=n; i++)
         if (col[i]==1 && !vis[i]) ++ans1,dfs(i,0);
     cout<<min(ans1,ans0);
     return 0;
 }*/