Nearest Common Ancestors

  1 #include <iostream>
  2 #include <cstdio>
  3 #include <cstring>
  4 #include <vector>
  5 using namespace std;
  6 #define MAXN 10001
  7 int n,fa[MAXN];
  8 int indegree[MAXN];
  9 int vis[MAXN];
 10 vector<int> hash[MAXN];
 11 struct Node{
 12    int v;
 13    int next;
 14    int id;
 15 }edge[MAXN*4];
 16 int head[MAXN],cnt;
 17 int ances[MAXN],ans[MAXN];//祖先
 18 void init(){
 19   memset(head,-1,sizeof(head));
 20   memset(ans,-1,sizeof(ans));
 21   cnt=0;
 22 }
 23 void add(int u,int v,int w){
 24     edge[cnt].v=v;
 25     edge[cnt].id=w;
 26     edge[cnt].next=head[u];
 27     head[u]=cnt++;
 28      edge[cnt].v=u;
 29     edge[cnt].id=w;
 30     edge[cnt].next=head[v];
 31     head[v]=cnt++;
 32 }
 33 
 34 void init(int n){
 35     for(int i=0;i<=n;i++){
 36         fa[i]=i;
 37         indegree[i]=0;
 38         vis[i]=0;
 39         ances[i]=0;
 40         hash[i].clear();
 41        // Qes[i].clear();
 42     }
 43 }
 44 int find(int x){
 45    int s;
 46    for(s=x;s!=fa[s];s=fa[s]);
 47    while(s!=x){
 48      int temp=fa[x];
 49      fa[x]=s;
 50      x=temp;
 51    }
 52    return s;
 53 }
 54 void unio(int x,int y){
 55     int fx=find(x),fy=find(y);
 56     if(fx==fy) return ;
 57      fa[fy]=fx;
 58 }
 59 void Tarjan(int u){
 60     ances[u]=u;
 61     int i,size = hash[u].size();
 62     for(i=0;i<size;i++){
 63         Tarjan(hash[u][i]);
 64         unio(u,hash[u][i]);
 65         ances[find(u)]=u;
 66     }
 67     vis[u]=1;
 68     for(int i=head[u];i!=-1;i=edge[i].next){
 69         int v=edge[i].v;
 70         if(vis[v]==1){
 71             ans[edge[i].id]=ances[find(v)];
 72             return;
 73         }
 74     }
 75 
 76     /*size = Qes[u].size();
 77     for(i=0;i<size;i++){
 78         if(vis[Qes[u][i]]==1){
 79             printf("%d\n",ances[find(Qes[u][i])]);
 80             return;
 81         }
 82     }*/
 83 }
 84 int main(){
 85     int t;
 86     int i,j;
 87     scanf("%d",&t);
 88     while(t--){
 89         scanf("%d",&n);
 90         init(n);
 91         int s,d;
 92         for(i=1;i<=n-1;i++){
 93             scanf("%d%d",&s,&d);
 94             hash[s].push_back(d);
 95             indegree[d]++;
 96         }
 97             scanf("%d%d",&s,&d);
 98             init();
 99             add(s,d,1);
100             add(d,s,1);
101         for(j=1;j<=n;j++){
102             if(indegree[j]==0){
103                 Tarjan(j);
104                 break;
105             }
106         }
107         printf("%d\n",ans[1]);
108     }
109     return 0;
110 }