[zjoi2004]嗅探器

#include<iostream>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<string>
#include<ctime>
#include<cmath>
#include<set>
#include<map>
#include<queue>
#include<algorithm>
#include<iomanip>
using namespace std;
#define FILE "dealing"
#define up(i,j,n) for(int i=(j);i<=(n);i++)
#define pii pair<int,int>
#define LL int
#define mem(f,g) memset(f,g,sizeof(f))
namespace IO{
	char buf[1<<15],*fs,*ft;
	int gc(){return (fs==ft&&(ft=(fs=buf)+fread(buf,1,1<<15,stdin),fs==ft))?-1:*fs++;}
	int read(){
		int ch=gc(),f=0,x=0;
		while(ch<‘0‘||ch>‘9‘){if(ch==‘-‘)f=1;ch=gc();}
		while(ch>=‘0‘&&ch<=‘9‘){x=(x<<1)+(x<<3)+ch-‘0‘;ch=gc();}
		return f?-x:x;
	}
	int readint(){
		int ch=getchar(),f=0,x=0;
		while(ch<‘0‘||ch>‘9‘){if(ch==‘-‘)f=1;ch=getchar();}
		while(ch>=‘0‘&&ch<=‘9‘){x=(x<<1)+(x<<3)+ch-‘0‘;ch=getchar();}
		return f?-x:x;
	}
}using namespace IO;
const int maxn=101000,inf=100000000;
int n,m;
int s,t,x,y;
struct node{
	int y,next;
}e[maxn<<1];
int len=0,linkk[maxn];
void insert(int x,int y){e[++len].y=y;e[len].next=linkk[x];linkk[x]=len;}
int pre[maxn],low[maxn],fa[maxn],dfs_clock=0;
int ans=inf;
void updata(int x){if(x!=s&&x!=t)if(ans>x)ans=x;}
int tarjan(int x){
	int flag=0,h;
	if(x==t)flag=1;
	low[x]=pre[x]=++dfs_clock;
	for(int i=linkk[x];i;i=e[i].next){
		if(e[i].y==fa[x])continue;
		fa[e[i].y]=x;
		if(!pre[e[i].y]){
			if(h=tarjan(e[i].y))flag=1;
			if(h&&low[e[i].y]>=pre[x])updata(x);
			if(low[e[i].y]<low[x])low[x]=low[e[i].y];
		}
		else if(pre[e[i].y]<low[x])low[x]=pre[e[i].y];
	}
	return flag;
}
int main(){
	freopen(FILE".in","r",stdin);
	freopen(FILE".out","w",stdout);
	n=read();
	while(true){
		x=read(),y=read();
		if(!x&&!y)break;
		insert(x,y);insert(y,x);
	}
	s=read(),t=read();
	tarjan(s);
	if(ans!=inf)printf("%d\n",ans);
	else printf("No solution\n");
	return 0;
}