本题是LCA问题的模板题，用tarjan算法来解决。
#include<iostream>
#include<vector>
#include<cstdio>
#include<string.h>
using namespace std;
const int maxn=10003;
vector<int> son[maxn],que[maxn];
int pre[maxn],ance[maxn];
bool vs[maxn];
int findx(int a)
{return (a==pre[a])?a:(pre[a]=findx(pre[a]));
}
void join(int a,int b)
{int fa=findx(a);int fb=findx(b);if(fa!=fb)pre[fa]=fb;
}void LCA(int root)
{ance[root]=root;for(int i=0;i<son[root].size();i++){LCA(son[root][i]);join(root,son[root][i]);ance[findx(son[root][i])]=root;}vs[root]=true;for(int i=0;i<que[root].size();i++){if(vs[que[root][i]]){printf("%d\n",ance[findx(que[root][i])]);return;}}
}int main()
{int t,n,from,to,head;scanf("%d",&t);while(t--){scanf("%d",&n);memset(ance,0,sizeof(pre));memset(vs,0,sizeof(vs));for(int i=1;i<=n;i++){pre[i]=i;son[i].clear();que[i].clear();}for(int i=1;i<n;i++){scanf("%d%d",&from,&to);son[from].push_back(to);vs[to]=true;}for(head=1;head<=n;head++)if(vs[head]==false)break;int a,b;scanf("%d%d",&a,&b);que[a].push_back(b);que[b].push_back(a);memset(vs,0,sizeof(vs));LCA(head);}return 0;
}