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2020-01-28 01:17:00
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求仙人掌直径

#include<bits/stdc++.h>
namespace ZDY{
    #pragma GCC optimize(3)
    #define il __inline__ __attribute__ ((always_inline))
    #define register
    #define ll long long
    #define ull unsigned long long
    #define db double
    #define sht short
    #define MB template <class T>il
    #define Fur(i,x,y) for(int i(x);i<=y;++i)
    #define Fdr(i,x,y) for(int i(x);i>=y;--i)
    #define fl(i,x) for(int i(head[x]),to;to=e[i].to,i;i=e[i].nxt)
    #define clr(x,y) memset(x,y,sizeof(x))
    #define cpy(x,y) memcpy(x,y,sizeof(x))
    #define fin(s) freopen(s".in","r",stdin)
    #define fout(s) freopen(s".out","w",stdout)
    #define fcin ios::sync_with_stdio(false)
    #define l2(n) ((int)(log2(n)))
    #define inf 2122219134
    MB T ABS(T x){return x>0?x:-x;}
    MB T MAX(T x,T y){return x>y?x:y;}
    MB T MIN(T x,T y){return x<y?x:y;}
    MB T GCD(T x,T y){return y?GCD(y,x%y):x;}
    MB void SWAP(T &x,T &y){T t=x;x=y;y=t;}
}using namespace ZDY;using namespace std;
namespace IO{const int str=1<<20;static char in_buf[str],*in_s,*in_t;bool __=0;il char gc(){return (in_s==in_t)&&(in_t=(in_s=in_buf)+fread(in_buf,1,str,stdin)),in_s==in_t?__=1,EOF:*in_s++;}il void in(string &ch){ch.clear();if(__)return;char c;while((c=gc())!=EOF&&isspace(c));if(c==EOF){__=1;return;}ch+=c;while((c=gc())!=EOF&&!isspace(c))ch+=c;if(c==EOF)__=1;}il void in(char &ch){if(__)return;char c;while((c=gc())!=EOF&&isspace(c));if(c==EOF)__=1;else ch=c;}il void in(char *ch){*ch='\0';if(__)return;char c;while((c=gc())!=EOF&&isspace(c));if(c==EOF){__=1;return;}*ch=c;ch++;while((c=gc())!=EOF&&!isspace(c))*ch=c,ch++;if(c==EOF)__=1;*ch='\0';}template<typename T>il void in(T &x){if(__)return;char c=gc();bool f=0;while(c!=EOF&&(c<'0'||c>'9'))f^=(c=='-'),c=gc();if(c==EOF){__=1;return;}x=0;while(c!=EOF&&'0'<=c&&c<='9')x=x*10+c-48,c=gc();if(c==EOF)__=1;if(f)x=-x;}template<typename T,typename ... arr>il void in(T &x,arr & ... y){in(x),in(y...);}const char ln='\n';static char out_buf[str],*out_s=out_buf,*out_t=out_buf+str;il void flush(){fwrite(out_buf,1,out_s-out_buf,stdout);out_s=out_buf;}il void pt(char c){(out_s==out_t)?(fwrite(out_s=out_buf,1,str,stdout),*out_s++=c):(*out_s++=c);}il void out(const char* s){while(*s)pt(*s++);}il void out(char* s){while(*s)pt(*s++);}il void out(char c){pt(c);}il void out(string s){for(int i=0;s[i];i++)pt(s[i]);}template<typename T>il void out(T x){if(!x){pt('0');return;}if(x<0)pt('-'),x=-x;char a[50],t=0;while(x)a[t++]=x%10,x/= 10;while(t--)pt(a[t]+'0');}template<typename T,typename ... arr>il void out(T x,arr & ... y){out(x),out(y...);}}using namespace IO;
const int N=100011;
int n,m,cnt=1,head[N],ans=0,f[N],d[N];
struct edge{
    int to,nxt;
}e[N<<1];
il void add(int x,int y){
    e[++cnt].to=y;e[cnt].nxt=head[x];head[x]=cnt;
    e[++cnt].to=x;e[cnt].nxt=head[y];head[y]=cnt;
}
int dfn[N],low[N],sz=0,fa[N],a[N],q[N];
void solve(int x,int y){
    int all=d[y]-d[x]+1,j=all;
    for(int i=y;i!=fa[x];i=fa[i])a[j--]=f[i];
    Fur(i,1,all)a[i+all]=a[i];
    int l=0,r=0;q[0]=1;
    Fur(i,2,all*2){
        while(l<=r&&i-q[l]>(all>>1))++l;
        ans=MAX(ans,a[i]+a[q[l]]+i-q[l]);
        while(l<=r&&a[q[r]]-q[r]<=a[i]-i)--r;
        q[++r]=i;
    }
    Fur(i,2,all)f[x]=MAX(f[x],a[i]+MIN(i-1,all-i+1));
}
void tarjan(int x,int la){
    dfn[x]=low[x]=++sz;
    fl(i,x)if(i!=la){
        if(!dfn[to]){
            fa[to]=x;d[to]=d[x]+1;
            tarjan(to,i^1);
            low[x]=MIN(low[x],low[to]);
            if(low[to]>dfn[x]){
                ans=MAX(ans,f[x]+f[to]+1);
                f[x]=MAX(f[x],f[to]+1);
            }
        }
        else low[x]=MIN(low[x],dfn[to]);
    }

    fl(i,x)if(fa[to]!=x&&dfn[to]>dfn[x])solve(x,to);
}
il void init(){
    in(n,m);
    while(m--){
        int t,x,y;in(t,x);
        while(--t)in(y),add(x,y),x=y;
    }
}
int main(){
    init();
    tarjan(1,0);
    printf("%d\n",ans);
}
LG 4244 [SHOI2008]仙人掌图-II
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