1 #include <cstdio>
 2 #include <cstring>
 3 #include <iostream>
 4 #include <algorithm>
 5 #include <vector>
 6 using namespace std;
 7 typedef long long ll;
 8 #define MS(a) memset(a,0,sizeof(a))
 9 #define MP make_pair
10 #define PB push_back
11 const int INF = 0x3f3f3f3f;
12 const ll INFLL = 0x3f3f3f3f3f3f3f3fLL;
13 inline ll read(){
14     ll x=0,f=1;char ch=getchar();
15     while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}
16     while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}
17     return x*f;
18 }
19 //
20 const int maxn = 1e4+10;
21
22 vector<int> g[maxn];
23 int f[maxn],vis[maxn],fa[maxn];
24 int q1,q2;
25 int a[maxn];
26
27 int find(int x){
28     return fa[x]==x ? x : fa[x]=find(fa[x]);
29 }
30
31 void Union(int x,int y){
32     int p1=find(x),p2=find(y);
33     if(p1 == p2) return ;
34     fa[p1] = p2;
35 }
36
37 void dfs(int u){
38     for(int i=0; i<(int)g[u].size(); i++){
39         int v = g[u][i];
40         dfs(v);
41         Union(u,v); // 合并的时候是u合并到v上，u的父亲是v，利于下面寻找祖先，也就是所有u的父亲的祖先都是u
42         a[find(u)] = u; // u的所有孩子的祖先都是u
43     }
44     vis[u] = 1;
45     if(q1==u && vis[q2]) printf("%d\n",a[find(q2)]);
46     if(q2==u && vis[q1]) printf("%d\n",a[find(q1)]);
47     return ;
48 }
49
50 int main(){
51     int T = read();
52     while(T--){
53         int n = read();
54         for(int i=0; i<=n; i++) {
55             g[i].clear();
56             f[i] = 0;
57             fa[i] = i;
58             vis[i] = 0;
59             a[i] = 0;
60         }
61         int u,v;
62         for(int i=1; i<n; i++){
63             scanf("%d%d",&u,&v);
64             f[v] = 1;
65             g[u].push_back(v);
66         }
67         cin >> q1 >> q2;
68         int i;
69         for(i=1; i<=n; i++)
70             if(f[i]==0) break;
71         dfs(i);
72     }
73
74     return 0;
75 }

 1 #include <cstdio>
 2 #include <cstring>
 3 #include <iostream>
 4 #include <algorithm>
 5 #include <vector>
 6 using namespace std;
 7 typedef long long ll;
 8 #define MS(a) memset(a,0,sizeof(a))
 9 #define MP make_pair
10 #define PB push_back
11 const int INF = 0x3f3f3f3f;
12 const ll INFLL = 0x3f3f3f3f3f3f3f3fLL;
13 inline ll read(){
14     ll x=0,f=1;char ch=getchar();
15     while(ch<'0'||ch>'9'){if(ch=='-')f=-1;ch=getchar();}
16     while(ch>='0'&&ch<='9'){x=x*10+ch-'0';ch=getchar();}
17     return x*f;
18 }
19 //
20 const int maxn = 1e4+10;
21
22 vector<int> g[maxn];
23 int d[maxn],p[maxn],f[maxn];
24
25 void dfs(int u,int fa,int de){
26     p[u] = fa;
27     d[u] = de;
28     for(int i=0; i<(int)g[u].size(); i++){
29         int v = g[u][i];
30         if(v == fa) continue;
31         dfs(v,u,de+1);
32     }
33 }
34
35 int lca(int u,int v){
36     while(d[u] > d[v]) u = p[u];
37     while(d[v] > d[u]) v = p[v];
38     while(u != v){
39         u = p[u];
40         v = p[v];
41     }
42
43     return u;
44 }
45
46 int main(){
47     int T = read();
48     while(T--){
49         int n = read();
50         for(int i=0; i<=n; i++) {
51             g[i].clear();
52             f[i] = 0; d[i] = 0; p[i] = 0;
53         }
54         int u,v;
55         for(int i=1; i<n; i++){
56             scanf("%d%d",&u,&v);
57             f[v] = 1;
58             g[u].push_back(v);
59         }
60         int i;
61         for(i=1; i<=n; i++)
62             if(f[i]==0) break;
63         dfs(i,-1,0);
64         cin >> u >> v;
65         cout << lca(u,v) << endl;
66     }
67
68     return 0;
69 }