洛谷 P4178 Tree

题链

首先点分治；
由于求解的是小于等于k的个数，于是开一个树状数组维护前缀和即可；

#include<bits/stdc++.h>
using namespace std;
#define LL long long 
#define MAXN 20009
#define MS 40009

int n,m;
struct node{
	int to,val;
};
vector<node > vc[MS];
int sz[MS],w[MS];
int rt,tr_size; 
int del[MS];
int dis[MS];
int dislist[MS] ,cntd;
int ext[MAXN];
queue<int > Q;
LL ans;
int p[MAXN]; // 树状数组 

int lowbit(int x){
	return x&(-x);
}

void add(int pos,int val){
	for(;pos<=20000;pos+=lowbit(pos)) p[pos] += val;
}

int get_sum(int pos){
	int ans = 0;
	for(;pos;pos-=lowbit(pos)) ans += p[pos];
	return ans;
}

void get_rt(int u,int f){
	sz[u] = 1;
	w[u] = 0;
	for(auto &nb:vc[u]){
		int v = nb.to;
		if(v != f && !del[v]){
			get_rt(v,u);
			sz[u] += sz[v];
			w[u] = max(w[u],sz[v]);
		} 
	}
	w[u] = max(w[u],tr_size-sz[u]);
	if(w[u] < w[rt]) rt = u;
}

void get_dis(int u,int f){
	if(dis[u] <= m) dislist[++cntd] = dis[u];
	for(auto &nb:vc[u]){
		int v = nb.to;
		int val = nb.val;
		if(v != f && !del[v]){
			dis[v] = dis[u] + val;
			get_dis(v,u);
		}
	}
}

void cal(int u){
	ext[0]++;
	add(1,1); // +1方便树状数组维护 
	for(auto &nb:vc[u]){
		int v = nb.to;
		int val = nb.val;
		if(!del[v]){
			cntd = 0;
			dis[v] = val;
			get_dis(v,u);
			
			for(int i=1;i<=cntd;i++){
				ans += get_sum(m-dislist[i]+1); // 树状数组求前缀和 
			}
			
			for(int i=1;i<=cntd;i++){
				ext[dislist[i]]++;
				add(dislist[i]+1,1);
				Q.push(dislist[i]);
			}
		}
	}
	ext[0]--;
	add(1,-1);
	while(!Q.empty()){
		ext[Q.front()]--;
		add(Q.front()+1,-1);
		Q.pop();
	}
}

void divide(int u){
	del[u] = 1;
	cal(u);
	for(auto &nb:vc[u]){
		int v = nb.to;
		if(!del[v]){
			w[rt = 0] = tr_size = sz[v];
			get_rt(v,0);
			get_rt(rt,0);
			divide(rt);
		}
	}
}

void solve(){
	cin >> n;
	for(int i=2;i<=n;i++){
		int u,v,val;
		cin >> u >> v >> val;
		vc[u].push_back({v,val});
		vc[v].push_back({u,val}); 
	}
	cin >> m;
	w[rt = 0] = tr_size = n;
	get_rt(1,0);
	get_rt(rt,0);
	divide(rt);
	
	cout << ans << "\n";
} 

int main(){
	ios::sync_with_stdio(false);
	int ce;
//	cin >> ce;
	ce = 1;
	while(ce--){
		solve();
	}
	
	return 0;
}

标签：rt,vc,洛谷,val,int,Tree,ce,MS,P4178
来源： https://www.cnblogs.com/Tecode/p/15043316.html