P6626

Sol

点分治板子，写一些细节：

1. 以下代码中，对于每个重心 $rt$，每次统计所有过 $rt$ 的链对所有答案的贡献。
2. 每一条链可以和子树外的所有链造成贡献，而并非前面枚举到的链。

Code

#include <bits/stdc++.h>
#define x first
#define y second
#define pb push_back
#define pf push_front
#define desktop "C:\\Users\\incra\\Desktop\\"
#define IOS ios :: sync_with_stdio (false),cin.tie (0),cout.tie (0)
using namespace std;
typedef long long LL;
typedef unsigned long long ULL;
typedef pair <int,int> PII;
const int dx[] = {1,0,-1,0},dy[] = {0,-1,0,1};
template <typename T1,typename T2> bool tomax (T1 &x,T2 y) {
	if (y > x) return x = y,true;
	return false;
}
template <typename T1,typename T2> bool tomin (T1 &x,T2 y) {
	if (y < x) return x = y,true;
	return false;
}
LL power (LL a,LL b,LL p) {
	LL ans = 1;
	while (b) {
		if (b & 1) ans = ans * a % p;
		a = a * a % p;
		b >>= 1;
	}
	return ans;
}
int fastio = (IOS,0);
// #define endl '\n'
#define puts(s) cout << (s) << endl
const int N = 100010;
int n,m;
vector <int> g[N];
int rt;
int cnt[N];
vector <int> vp,vq;
int dist[N];
bool del[N];
vector <PII> qs[N];
int ans[N];
int get_size (int u,int fa) {
    int s = 1;
    for (int v : g[u]) {
        if (v == fa || del[v]) continue;
        s += get_size (v,u);
    }
    return s;
}
int get_root (int u,int fa,int tot) {
    int maxx = 0,s = 1;
    for (int v : g[u]) {
        if (v == fa || del[v]) continue;
        int t = get_root (v,u,tot);
        tomax (maxx,t);
        s += t;
    }
    tomax (maxx,tot - s);
    if (maxx <= tot / 2) rt = u;
    return s;
}
void get_dist (int u,int fa) {
    vq.pb (u);
    for (int v : g[u]) {
        if (v == fa || del[v]) continue;
        dist[v] = dist[u] + 1;
        get_dist (v,u);
    }
}
void calc (int u) {
    vq.clear ();
    dist[u] = 0;
    get_dist (u,-1);
    for (int x : vq) cnt[dist[x]]++;
    for (auto [k,id] : qs[u]) ans[id] += cnt[k];
    vp = vq;
    for (int v : g[u]) {
        if (del[v]) continue;
        vq.clear ();
        dist[v] = 1;
        get_dist (v,u);
        for (int x : vq) cnt[dist[x]]--;
        for (int x : vq) {
            for (auto [k,id] : qs[x]) {
                if (k - dist[x] >= 0) ans[id] += cnt[k - dist[x]];
            }
        }
        for (int x : vq) cnt[dist[x]]++;
    }
    for (int x : vp) cnt[dist[x]]--;
}
void solve (int _u) {
    get_root (_u,-1,get_size (_u,-1));
    int u = rt;
    calc (u);
    del[u] = 1;
    for (int v : g[u]) {
        if (del[v]) continue;
        solve (v);
    }
}
void mian () {
    cin >> n >> m;
    for (int i = 1;i <= n;i++) g[i].clear (),del[i] = 0,qs[i].clear ();
    for (int i = 1;i <= m;i++) ans[i] = 0;
    for (int i = 1;i <= n - 1;i++) {
        int a,b;
        cin >> a >> b;
        g[a].pb (b),g[b].pb (a);
    }
    for (int i = 1;i <= m;i++) {
        int x,y;
        cin >> x >> y;
        qs[x].pb ({y,i});
    }
    solve (1);
    for (int i = 1;i <= m;i++) cout << ans[i] << endl;
}
int main () {
	int T = 1;
	cin >> T;
	while (T--) mian ();
	return 0;
}