天呐，用cin只过两个点，用scanf就AC了，无语

#include<iostream>

using namespace std;

int f[1000001],jh[1000001];

void deal(int a){

jh[a]=1;

while(jh[a]!=a){

a=f[a];

jh[a]=1;

}

}

void work(int a){

while(!jh[a])a=f[a];

cout<<a;

}

int main(){

int i,n,a,b;

cin>>n;

memset(jh,0,sizeof(jh));

for(i=1;i<=n;i++)f[i]=i;

for(i=1;i<=n-1;i++){

scanf("%d%d",&a,&b);

f[b]=a;

}

cin>>a>>b;

deal(a);

work(b);

return 0;

}