CodeForces 337D Book of Evil

#include <bits/stdc++.h>
using namespace std;

#define rep(i,n) for (int i = 0; i < (n); ++i)
#define For(i,s,t) for (int i = (s); i <= (t); ++i)
#define rFor(i,t,s) for (int i = (t); i >= (s); --i)
#define foreach(i,c) for (__typeof(c.begin()) i = c.begin(); i != c.end(); ++i)
#define rforeach(i,c) for (__typeof(c.rbegin()) i = c.rbegin(); i != c.rend(); ++i)

#define pr(x) cout << #x << " = " << x << "  "
#define prln(x) cout << #x << " = " << x << endl

#define LOWBIT(x) ((x)&(-x))

#define ALL(x) x.begin(),x.end()
#define INS(x) inserter(x,x.begin())

#define ms0(a) memset(a,0,sizeof(a))
#define msI(a) memset(a,inf,sizeof(a))
#define msM(a) memset(a,-1,sizeof(a))

#define pii pair<int,int> 
#define piii pair<pair<int,int>,int> 
#define mp make_pair
#define pb push_back
#define fi first
#define se second

inline int gc(){
    static const int BUF = 1e7;
    static char buf[BUF], *bg = buf + BUF, *ed = bg;
    
    if(bg == ed) fread(bg = buf, 1, BUF, stdin);
    return *bg++;
} 

inline int ri(){
    int x = 0, f = 1, c = gc();
    for(; c<48||c>57; f = c=='-'?-1:f, c=gc());
    for(; c>47&&c<58; x = x*10 + c - 48, c=gc());
    return x*f;
}

typedef long long LL;
typedef unsigned long long uLL;
const int inf = 1e9 + 9;
const LL mod = 1e9 + 7;
const int maxN = 1e5 + 7;

struct Node{
    vector< int > next;
};

int n, m, d, ans;
int vis[maxN];
int ghost[maxN]; // 记录是否是幽灵 
int state[maxN]; // 标记是否可能存在书，值为2的时候才存在 
Node nodes[maxN];
queue< int > Q;

int g_1, g_2; // 最远幽灵点对 

// 返回与幽灵节点x距离最远的幽灵节点的幽灵节点序号 
int bfs_1(int x) {
    int dist = 1;
    int cnt = m - 1, cnt_1 = 1;
    if(cnt == 0) return x;
    while(!Q.empty()) Q.pop();
    ms0(vis);
    vis[x] = 1; 
    Q.push(x);
    
    while(!Q.empty()) {
        --cnt_1;
        if(dist > 2 * d) return -1;
        int tmp = Q.front();
        Q.pop();
        
        foreach(i, nodes[tmp].next) {
            if(vis[*i]) continue;
            vis[*i] = 1;
            if(ghost[*i] == 1) {
                --cnt;
                if(cnt == 0) return *i;
            }
            Q.push(*i);
        }
        if(cnt_1 == 0) {
            ++dist;
            cnt_1 = Q.size();
        }
    }
}

// 以幽灵节点x为中心，向外bfs，记录所有与x距离小于等于d的节点，自身节点也算 
void bfs_2(int x) {
    int dist = 1, cnt_1 = 1;
    while(!Q.empty()) Q.pop();
    ms0(vis);
    vis[x] = 1; 
    Q.push(x);
    ++state[x];
    
    while(!Q.empty()) {
        --cnt_1;
        if(dist > d) return;
        int tmp = Q.front();
        Q.pop();
        
        foreach(i, nodes[tmp].next) {
            if(vis[*i]) continue;
            vis[*i] = 1;
            ++state[*i];
            Q.push(*i);
        }
        if(cnt_1 == 0) {
            ++dist;
            cnt_1 = Q.size();
        }
    }
}

int main(){
    cin >> n >> m >> d;
    For(i, 1, m) {
        cin >> g_1;
        ghost[g_1] = 1;
    }
    
    For(i, 1, n-1) {
        int x, y;
        cin >> x >> y;
        nodes[x].next.push_back(y);
        nodes[y].next.push_back(x);
    }
    
    g_2 = bfs_1(g_1); 
    g_1 = bfs_1(g_2); 
    
    if(g_1 != -1 && g_2 != -1) {
        bfs_2(g_1);
        bfs_2(g_2);
        
        For(i, 1, n) if(state[i] == 2) ++ans;
    }
    
    cout << ans << endl;
    return 0;
}

/*
25 7 8
2 6 9 13 20 17 23
1 2
1 3
1 4
2 5
5 8
5 9
8 15
9 16
3 6
6 10
6 11
6 12
6 13
11 17
17 22
17 23
12 18
4 7
7 14
14 19
14 20
20 24
20 25
14 21

ans: 16
*/