1523：嗅探器

找割点，且去掉这个点后 S,T 两个点在2个块中

 

除了dfn[x]<=low[y] ,  还有 dfn[y]<=dfn[T ]

 

?

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#include <bits/stdc++.h> using namespace std ;  const int N=2*1e5+1,M=5*1e5+1;     int nxt[M],head[N],go[M],all;      void add(int a,int b){     all++; nxt[all]=head[a]; go[all]=b;      head[a]=all;  }  int n,m,low[N],dfn[N],ans=1e8,pool,S,T;      void tar(int x) {      dfn[x]=low[x]=++pool;      int y,i;      for(i=head[x];i;i=nxt[i]){          y=go[i];          if(!dfn[y]){             tar(y),low[x]=min(low[x],low[y]);             if(low[y]>=dfn[x]&&                 dfn[y]<=dfn[T]&&x!=S&&x!=T){                 ans=min(ans,x);             }            }          else low[x]=min(low[x],dfn[y]);      }  }    int main(){     cin>>n;     int x,y;     while(cin>>x>>y,x&&y){         add(x,y);add(y,x);     }     cin>>S>>T;     tar(S);     if(ans==1e8) cout<<"No solution";else cout<<ans; }