hdu2586 How far away ？--tarjan & LCA

原题链接：​​http://acm.hdu.edu.cn/showproblem.php?pid=2586​​

题意：n个点，编号1-n，接下来n-1行，每行三个数字表示两点之间的距离，题目是保证两点间不会出现两条可行的路，也就是不会有环；m条询问，每条询问两个数字a，b表示a，b的最短距离。

#define _CRT_SECURE_NO_DEPRECATE 

#include<iostream>
#include<vector>
#include<cstring>
#include<queue>
#include<stack>
#include<algorithm>
#include<cmath>
#define INF 99999999
#define eps 0.0001
using namespace std;

struct Edge
{
  int w;
  int v;
  int next;
};

int t;
int n, m;
int cnt;
Edge edge[40005 * 2];
int head[40005];
int x[40005];
int y[40005];
int z[40005];
int pre[40005];
int dist[40005];
bool vis[40005];

void add(int u, int v, int w)
{
  edge[cnt].w = w; edge[cnt].v = v; edge[cnt].next = head[u]; head[u] = cnt++;
  edge[cnt].w = w; edge[cnt].v = u; edge[cnt].next = head[v]; head[v] = cnt++;
}

int find(int x)
{
  if (pre[x] != x)
    return pre[x] = find(pre[x]);
  return x;
}

void tarjan(int u)
{
  vis[u] = 1;
  pre[u] = u;

  for (int i = 0; i < m; i++)
  { 
    if (x[i] == u&&vis[y[i]]) z[i] = find(y[i]);
    if (y[i] == u&&vis[x[i]]) z[i] = find(x[i]);
  }
  
  for (int i = head[u]; i != -1; i = edge[i].next)
  {
    int v = edge[i].v;
    if (!vis[v])
    {
      dist[v] = dist[u] + edge[i].w;
      tarjan(v);
      pre[v] = u;
    }
  }
}

int main()
{
  scanf("%d", &t);
  while (t--)
  {
    scanf("%d%d", &n, &m);

    int u, v, w;
    cnt = 0;
    memset(head, -1, sizeof(head));
    for (int i = 1; i < n; i++)
    {
      scanf("%d%d%d", &u, &v, &w);
      add(u, v, w);
    }

    for (int i = 0; i < m; i++)
    {
      scanf("%d%d", &u, &v);
      x[i] = u;
      y[i] = v;
    }

    memset(vis, 0, sizeof(vis));
    dist[1] = 0;
    tarjan(1);

    for (int i = 0; i < m; i++)
      printf("%d\n", dist[x[i]] + dist[y[i]] - 2 * dist[z[i]]);
  }

  return 0;
}