How far away ？

How far away ？

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

Time Limit: 2000/1000 MS (Java/Others)    Memory Limit: 32768/32768 K (Java/Others)
Total Submission(s): 25399    Accepted Submission(s): 10108

Problem Description

There are n houses in the village and some bidirectional roads connecting them. Every day peole always like to ask like this "How far is it if I want to go from house A to house B"? Usually it hard to answer. But luckily int this village the answer is always unique, since the roads are built in the way that there is a unique simple path("simple" means you can't visit a place twice) between every two houses. Yout task is to answer all these curious people.

 

Input

First line is a single integer T(T<=10), indicating the number of test cases.
  For each test case,in the first line there are two numbers n(2<=n<=40000) and m (1<=m<=200),the number of houses and the number of queries. The following n-1 lines each consisting three numbers i,j,k, separated bu a single space, meaning that there is a road connecting house i and house j,with length k(0<k<=40000).The houses are labeled from 1 to n.
  Next m lines each has distinct integers i and j, you areato answer the distance between house i and house j.

 

Output

For each test case,output m lines. Each line represents the answer of the query. Output a bland line after each test case.

 

Sample Input

2

3 2

1 2 10

3 1 15

1 2

2 3

2 2

1 2 100

1 2

2 1

 

Sample Output

10

25

100

100

 

Source

ECJTU 2009 Spring Contest

 

在JV dalao的指导下，终于学会了tarjan。。。

推荐一下他写的关于tarjan的文章，很详细：http://www.cnblogs.com/JVxie/p/4854719.html

模板题

#include <iostream>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <algorithm>
#include <string>
#include<vector>
#include <queue>
const int INF=0x3f3f3f3f;
#define maxn 40005
using namespace std;

int n,m;

vector<pair<int,int> >mp[maxn],book[maxn];
int fa[maxn];
int vis[maxn],dis[maxn];
int ans[maxn];

int Find(int x){
    int r=x,y;
    while(x!=fa[x]){
        x=fa[x];
    }
    while(r!=x){
        y=fa[r];
        fa[r]=x;
        r=y;
    }
    return x;
}

int join(int x,int y){
    int xx=Find(x);
    int yy=Find(y);
    if(xx!=yy){
        fa[yy]=xx;
        return true;
    }
    return false;
}

void tarjan(int x,int pre,int dist){
    int to,len,tmp;
    for(int i=0;i<mp[x].size();i++){
        to=mp[x][i].first;
        len=mp[x][i].second;
        tmp=dist+len;
        if(to!=pre&&!vis[to]){
            dis[to]=tmp;
            tarjan(to,x,tmp);
            vis[to]=1;
            join(x,to);
        }  
    }
    int pos;
    for(int i=0;i<book[x].size();i++){
        to=book[x][i].first;
        pos=book[x][i].second;
        if(vis[to]&&ans[pos]==-1){
            tmp=Find(to);
            ans[pos]=dis[to]+dis[x]-2*dis[tmp];
        }
    }
}

int main(){
    std::ios::sync_with_stdio(false);
    int T;
    cin>>T;
    while(T--){
        memset(vis,0,sizeof(vis));
        memset(ans,-1,sizeof(ans));
        memset(dis,0,sizeof(dis));
        for(int i=0;i<=n;i++){
            mp[i].clear();
            book[i].clear();
        }
        int x,y,z;
        cin>>n>>m;
        for(int i=0;i<=n;i++) fa[i]=i;
        for(int i=1;i<n;i++){
            cin>>x>>y>>z;
            mp[x].push_back(make_pair(y,z));
            mp[y].push_back(make_pair(x,z));
        }
        for(int i=1;i<=m;i++){
            cin>>x>>y;
            book[x].push_back(make_pair(y,i));
            book[y].push_back(make_pair(x,i));
        }
        tarjan(1,-1,0);
        for(int i=1;i<=m;i++){
            cout<<ans[i]<<endl;
        }
    }
    system("pause");
}