sicily 2377 Ants Colony 【LCA Tarjan算法】

　　周赛第二次的题目。传说中的最近公共祖先问题(LCA),用Tarjan算法可以过，但注意结果超int.

　　从题意可以看出这些colony之间构成了一棵树的关系，用Tarjan算法找出每对查询的顶点u,v的公共祖先ancestor,则u,v之间的距离就是dist[u]+dist[v]-2*dist[ancestor],关系显而易见，会者不难。

#include <iostream>
#include <stdio.h>
#include <vector>

using namespace std;
const int MAX = 100001;
struct Edge
{
    int end; int dis;
    Edge(int e,int d): end(e),dis(d) {}
};
struct Quer
{
    int v; int count;
    Quer(int vv,int c): v(vv),count(c) {}
};
vector<Edge> Graph[MAX];
vector<Quer> query[MAX];
bool vis[MAX];
int fa[MAX],n,q;
long long dist[MAX],ans[MAX];

void init()
{
    int i;

    for(i = 0;i <= n;++i) 
    {
        Graph[i].clear();
        dist[i] = 0;
        fa[i] = i;
        vis[i] = false;
    }
    for(i = 0;i <= MAX;++i) query[i].clear();
}

int Findset(int x)
{
    while(fa[x] != x)
    {
        x = fa[x];
    }

    return x;
}

void Tarjan(int now,long long cur_dis)
{
    int i,end,dis,v,count;

    vis[now] = true; dist[now] = cur_dis;  fa[now] = now;

    for(i = 0;i < query[now].size();++i)
    {
        v = query[now][i].v; count = query[now][i].count;

        if (vis[v])
        {
          ans[count] = dist[now]+dist[v]-2*dist[Findset(v)];
        }
    }

    for(i = 0;i < Graph[now].size();++i)
    {
        end = Graph[now][i].end; dis = Graph[now][i].dis;

        if(!vis[end])
        {
            Tarjan(end,cur_dis + dis);
            fa[end] = now;
        }
    }
}

int main()
{
    int i,u,v,d;

    while(scanf("%d",&n) && n)
    {
        init();

        for(i = 1;i < n;++i)
        {
            scanf("%d %d",&v,&d);
            Graph[v].push_back(Edge(i,d));
            Graph[i].push_back(Edge(v,d));
        }

        scanf("%d",&q);
        for(i = 0;i < q;++i)
        {
            scanf("%d %d",&u,&v);
            query[u].push_back(Quer(v,i));
            query[v].push_back(Quer(u,i));
        }

        Tarjan(0,0);

        for(i = 0;i < q;++i)
        {
            if(i) printf(" ");
            cout<<ans[i];
        }
        printf("\n");
    }

    return 0;
}