又写了一遍，发出来做个记录

 

#include
     
      #include
      
       #include
       
        using namespace std;#define N 500001int tot=1,front[N],to[N<<2],nxt[N<<2];int id,dfn[N],low[N];int fa[N],dep[N];int dp[N];int tmp[N],q[N];int ans;void read(int &x){    x=0; char c=getchar();    while(!isdigit(c)) c=getchar();    while(isdigit(c)) { x=x*10+c-'0'; c=getchar(); }}void add(int u,int v){    to[++tot]=v; nxt[tot]=front[u]; front[u]=tot;    to[++tot]=u; nxt[tot]=front[v]; front[v]=tot;}void circle(int x,int y){    int cnt=dep[y]-dep[x]+1;    int now=y;    while(dfn[fa[now]]>=dfn[x]) tmp[cnt--]=now,now=fa[now];    tmp[cnt]=now;    cnt=dep[y]-dep[x]+1;    int nn=cnt;    for(int i=1;i<=cnt;++i) tmp[++nn]=tmp[i];    int h=0,t=0;    for(int i=1;i<=nn;++i)    {        while(h
        
         cnt/2) h++;        if(h
         
          dp[tmp[q[t-1]]]-q[t-1]) t--;        q[t++]=i;    }    for(int i=2;i<=cnt;++i) dp[x]=max(dp[x],dp[tmp[i]]+min(i-1,cnt-i+1));}void tarjan(int x,int y){    dfn[x]=low[x]=++id;    int t;    for(int i=front[x];i;i=nxt[i])    {        if(i==(y^1)) continue;        t=to[i];        if(!dfn[t])        {            fa[t]=x;            dep[t]=dep[x]+1;            tarjan(t,i);            low[x]=min(low[x],low[t]);        }        else low[x]=min(low[x],dfn[t]);        if(dfn[x]