学步园

#include <cstdio>
#include <cctype>
#include <vector>
#include <cstring>
#include <iostream>
#include <algorithm>
using namespace std;

const int N    = 491520+10;
const int maxn = 15;
struct node{
int s,x;
node(int s=0,int x=0):s(s),x(x){}
}q[N],act[N];
int vis[1<<maxn][maxn],n,m,s,t,fa[N],dist[N]; //491520
vector<int> G[maxn];

void print_ans(int front_){
if(fa[front_]==-1) return ;
print_ans(fa[front_]);
printf("%d %d\n",act[front_].s+1,act[front_].x+1);
}
int bfs(node& S){
memset(vis,0,sizeof(vis));
q[0] = S; dist[0] = 0; fa[0] = -1;
int front_=0,rear=1;
while(front_ < rear){
    node& u = q[front_];
    if(u.x == t-1){
          printf("%d\n",dist[front_]);
          print_ans(front_); return 1;
    }
    for(int i=0;i<n;i++)if((u.s>>i)&1){
           for(int j=0;j<G[i].size();j++)if(!((u.s>>G[i][j])&1)){
                   node& v = q[rear];
                   v = u;
                   if(v.x == i) v.x = G[i][j];
                   v.s&=~(1<<i); v.s|=(1<<G[i][j]);
                   if(!vis[v.s][v.x]){
                        vis[v.s][v.x] = 1;
                        fa[rear] = front_;
                        act[rear] = node(i,G[i][j]);
                        dist[rear] = dist[front_] + 1;
                        rear++;
                   }
           }
    }
    front_++;
}
}
int main()
{
    int T,kase=1;
    scanf("%d",&T);
    while(T--){
        scanf("%d %d %d %d",&n,&m,&s,&t);
        node S;   S.s|=(1<<(s-1)); S.x=s-1;
        for(int i=1;i<=m;i++) {
            int x;
            scanf("%d",&x); S.s|=(1<<(x-1));
        }
        for(int i=0;i<n;i++) G[i].clear();
        for(int i=1;i<n;i++){
             int x,y;
            scanf("%d %d",&x,&y);
            G[x-1].push_back(y-1);
            G[y-1].push_back(x-1);
        }
        printf("Case %d: ",kase++);
        int ans = bfs(S);
        printf("\n");
    }
    return 0;
}