题目链接：HDU2586

【题意】给定一颗有权树，求任意两点之间的最短路。

【分析】是LCA的基本应用，树上任意两点(u,v)的最短路 = dis[u]+dis[v]-2*dis[LCA(u,v)];dis[i]为根到i的距离，我写了两个版本，离线和用线段树的在线(因为这里查询很少，用线段树比ST快，因为线段树初始化为O(n)的，ST是O(nlogn)的)

【AC CODE(离线)】46ms

#include <cstdio>
#include <cstring>
#include <cctype>
#include <cmath>
#include <map>
//#include <unordered_map>
#include <queue>
#include <stack>
#include <vector>
#include <string>
#include <algorithm>
using namespace std;
typedef long long LL;
#define rep(i,a,n) for(int i = a; i < n; i++)
#define repe(i,a,n) for(int i = a; i <= n; i++)
#define per(i,n,a) for(int i = n; i >= a; i--)
#define clc(a,b) memset(a,b,sizeof(a))
const int INF = 0x3f3f3f3f, MAXN = 40000+10, MAXM = 40000*2+10;
int head[MAXN], tol, to[MAXM], nxt[MAXM], cost[MAXM];
struct Q{
	int v, id;
};
vector<Q> q[MAXN];

void add_edge(int u, int v, int c)
{
	nxt[tol] = head[u], to[tol] = v, cost[tol] = c;
	head[u] = tol++;
}
int f[MAXN];
int find(int x){return f[x] == x?x:f[x] = find(f[x]);}
void bing(int a, int b)
{
	int x = find(a), y = find(b);
	if(x != y) f[y] = x;
}
bool vis[MAXN];
int ans[200+10], dis[MAXN];
void lca(int u)
{
	vis[u] = true;
	for(int i = head[u]; ~i; i = nxt[i])
	{
		int v = to[i];
		if(vis[v]) continue;
		dis[v] = dis[u]+cost[i];
		lca(v);
		bing(u,v);
	}
	int sz = q[u].size();
	rep(i,0,sz)
	{
		int v = q[u][i].v;
		if(vis[v])
		{
			int fa = find(v);
			ans[q[u][i].id] = dis[u]+dis[v]-2*dis[fa];
		}
	}
}
int main()
{
#ifdef SHY
	freopen("d:\\1.txt", "r", stdin);
#endif
	int t;
	scanf("%d%*c", &t);
	while(t--)
	{
		int n,m;
		scanf("%d %d%*c", &n, &m);
		clc(head,-1);
		tol = 0;
		rep(i,1,n)
		{
			int u,v,c;
			scanf("%d %d %d%*c", &u, &v, &c);
			add_edge(u,v,c);
			add_edge(v,u,c);
		}
		repe(i,1,n) q[i].clear(), f[i] = i;
		rep(i,0,m)
		{
			int u,v;
			scanf("%d %d%*c", &u, &v);
			q[u].push_back(Q{v,i});
			q[v].push_back(Q{u,i});
		}
		clc(vis,0);
		dis[1] = 0;
		lca(1);
		rep(i,0,m) printf("%d\n", ans[i]);
	}
	return 0;
}

【AC CODE（在线）】31ms

#include <cstdio>
#include <cstring>
#include <cctype>
#include <cmath>
#include <map>
//#include <unordered_map>
#include <queue>
#include <stack>
#include <vector>
#include <string>
#include <algorithm>
using namespace std;
typedef long long LL;
#define rep(i,a,n) for(int i = a; i < n; i++)
#define repe(i,a,n) for(int i = a; i <= n; i++)
#define per(i,n,a) for(int i = n; i >= a; i--)
#define clc(a,b) memset(a,b,sizeof(a))
const int INF = 0x3f3f3f3f, MAXN = 40000+10, MAXM = 40000*2+10;
int head[MAXN], tol, to[MAXM], nxt[MAXM], cost[MAXM];

void add_edge(int u, int v, int c)
{
	nxt[tol] = head[u], to[tol] = v, cost[tol] = c;
	head[u] = tol++;
}
int dis[MAXN],ft[MAXN],d[MAXN<<1],num[MAXN<<1],cnt;
bool vis[MAXN];
void dfs(int u, int deep)
{
	ft[u] = cnt;
	d[cnt] = deep, num[cnt++] = u;
	vis[u] = true;
	for(int i = head[u]; ~i; i = nxt[i])
	{
		int v = to[i];
		if(vis[v]) continue;
		dis[v] = dis[u]+cost[i];
		dfs(v,deep+1);
		d[cnt] = deep, num[cnt++] = u;
	}
}
int mi_num[MAXN<<2];
inline int id(int x, int y){return x+y|x!=y;}
inline void push_up(int x, int y, int m)
{
	int u = id(x,y), l = id(x,m), r = id(m+1,y);
	if(d[mi_num[l]] < d[mi_num[r]]) mi_num[u] = mi_num[l];
	else mi_num[u] = mi_num[r];
}
void bulid(int x, int y)
{
	if(x == y)
	{
		mi_num[id(x,y)] = x;
		return;
	}
	int m = (x+y)>>1;
	bulid(x,m);
	bulid(m+1,y);
	push_up(x,y,m);
}
int query(int x, int y, int ql, int qr)
{
	if(ql <= x && y <= qr) return mi_num[id(x,y)];
	int m = (x+y)>>1, ans, mi = INF;
	if(ql <= m)
	{
		int p = query(x,m,ql,qr);
		if(mi > d[p]) mi = d[p], ans = p;
	}
	if(qr > m)
	{
		int p = query(m+1,y,ql,qr);
		if(mi > d[p]) mi = d[p], ans = p;
	}
	return ans;
}
void lca_init()
{
	cnt = 0;
	dis[1] = 0;
	clc(vis,0);
	dfs(1,0);
	bulid(0,cnt-1);
}
int lca_query(int x, int y)
{
	int l = ft[x], r = ft[y];
	if(l > r) swap(l,r);
	int fa = num[query(0,cnt-1,l,r)];
	return dis[x]+dis[y]-2*dis[fa];
}
int main()
{
#ifdef SHY
	freopen("d:\\1.txt", "r", stdin);
#endif
	int t;
	scanf("%d%*c", &t);
	while(t--)
	{
		int n,m;
		scanf("%d %d%*c", &n, &m);
		clc(head,-1);
		tol = 0;
		rep(i,1,n)
		{
			int u,v,c;
			scanf("%d %d %d%*c", &u, &v, &c);
			add_edge(u,v,c);
			add_edge(v,u,c);
		}
		lca_init();
		rep(i,0,m)
		{
			int u,v;
			scanf("%d %d%*c", &u, &v);
			printf("%d\n", lca_query(u,v));
		}
	}
	return 0;
}