hdu 2586 How far away？(树链剖分+前缀和）

Posted 2021-06-15 jpphy0

How far away？

http://acm.hdu.edu.cn/showproblem.php?pid=2586

分析

• 链式前向星
• 树链剖分、最近公共祖先（LCA）
• 根到叶子路径的前缀和

代码

// hdu 2586 How far away？
#include <bits/stdc++.h>
using namespace std;
#define MXN 40005
int h[MXN], fa[MXN], top[MXN], siz[MXN], d[MXN], son[MXN];
long long sum[MXN];
int n, m, cnt, ans;
struct Edge{int to = -1, nxt = -1, w;}e[MXN<<1];
void add(int u, int v, int w){
	e[cnt].w = w;
	e[cnt].to = v;
	e[cnt].nxt = h[u];
	h[u] = cnt++;
}
void dfs1(int x, int pre){
	fa[x] = pre, d[x] = d[pre]+1;
	siz[x] = 1, son[x] = 0;
	for(int i = h[x]; ~i; i = e[i].nxt){// i != -1
		int to = e[i].to;
		if(to == pre) continue;
		sum[to] = sum[x]+e[i].w;
		dfs1(to, x);
		siz[x] += siz[to];
		if(siz[son[x]] < siz[to]) son[x] = to;
	}
}
void dfs2(int x, int topx){
	top[x] = topx;
	if(son[x] == 0) return;
	dfs2(son[x], topx);
	for(int i = h[x]; ~i; i = e[i].nxt){
		int to = e[i].to;
		if(to == fa[x] || to == son[x]) continue;
		dfs2(to, to);
	}
}
int lca(int u, int v){
	while(top[u] != top[v]){
		// if(d[u] > d[v]) u = fa[top[u]]; // 错误，测试如下
		/*
		7 1
		1 2 4
		2 3 5
		2 4 6
		3 5 8
		4 6 9
		6 7 4
		5 7
		*/
		if(d[top[u]] > d[top[v]]) u = fa[top[u]];
		else v = fa[top[v]];
	}
	return d[u] > d[v] ? v : u;
}
int main() {
	int u, v, w, t, tmp;
	scanf("%d", &t);
	while (t--) {
		memset(h, -1, sizeof h);
		cnt = 1;
		scanf("%d%d", &n, &m);
		for(int i = 1; i < n; ++i){
			scanf("%d%d%d", &u, &v, &w);
			add(u, v, w), add(v, u, w);
		}
		sum[1] = 0, d[0] = 0;
		dfs1(1, 0), dfs2(1, 1);
		while(m--){
			scanf("%d%d", &u, &v);
			tmp = lca(u, v);
			printf("%lld\\n", sum[u]-sum[tmp]-sum[tmp]+sum[v]);			
		}
		if(t!=0) printf("\\n");
	}
	return 0;
}