COGS 1439. [NOIP2013]货车运输

★★☆   输入文件：truck.in   输出文件：truck.out   简单对比
时间限制：1 s   内存限制：128 MB

【题目描述】

 

【来源】

CCF全国信息学奥林匹克联赛（NOIP2013）复赛Day1

 

最大生成树+LCA

屠龙宝刀点击就送

#include <algorithm>
#include <ctype.h>
#include <cstdio>
#define N 20005
#define M 50005
using namespace std;
void read(int &x)
{
    x=0;char ch;
    for(;!isdigit(ch);ch=getchar());
    for(;isdigit(ch);ch=getchar()) x=x*10+ch-'0';
}
struct E
{
    int x,y,z;
    bool operator<(E a)const
    {
        return z>a.z;
    }
}e[M<<1];
struct Edge
{
    int next,to,dis;
    Edge (int next=0,int to=0,int dis=0) :next(next),to(to),dis(dis) {}
}edge[M<<1];
bool vis[N];
int dep[N],pre[N],val[N],fa[N],dis[N],q,head[M],cnt,n,m;
int find_(int x)
{
    return fa[x]==x?x:fa[x]=find_(fa[x]);
}
void insert(int u,int v,int w)
{
    edge[++cnt]=Edge(head[u],v,w);
    head[u]=cnt;
}
int min(int a,int b) {return a>b?b:a;}
void dfs(int x)
{
    vis[x]=true;
    for(int u=head[x];u;u=edge[u].next)
    {
        int v=edge[u].to;
        if(!vis[v])
        {
            pre[v]=x;
            val[v]=edge[u].dis;
            dep[v]=dep[x]+1;
            dfs(v);
        }
    }
}
void swap(int &a,int &b)
{
    int tmp=b;
    b=a;
    a=tmp;
}
int getans(int a,int b)
{
    int min1=0x7fffffff,min2=0x7fffffff;
    if(dep[a]<dep[b]) swap(a,b);
    while(dep[a]>dep[b])
    {
        min1=min(min1,val[a]);
        a=pre[a];
    }
    while(a!=b)
    {
        min1=min(min1,val[a]);
        a=pre[a];
        min2=min(min2,val[b]);
        b=pre[b];
    }
    return min(min1,min2);
}
int main()
{
    freopen("truck.in","r",stdin);
    freopen("truck.out","w",stdout);
    read(n);
    read(m);
    for(int x,y,z,i=1;i<=m;i++)
    {
        read(e[i].x);
        read(e[i].y);
        read(e[i].z);
    }
    for(int i=1;i<=n;i++) fa[i]=i;
    sort(e+1,e+1+m);
    int num=0;
    for(int i=1;i<=m;i++)
    {
        int fx=find_(e[i].x),fy=find_(e[i].y);
        if(fx!=fy)
        {
            num++;
            fa[fy]=fx;
            insert(e[i].x,e[i].y,e[i].z);
            insert(e[i].y,e[i].x,e[i].z);
            if(num==n-1) break;
        }
    }
    for(int i=1;i<=n;i++) if(!vis[i]) dfs(i);
    read(q);
    for(int x,y;q--;)
    {
        read(x);
        read(y);
        int fx=find_(x),fy=find_(y);
        if(fx!=fy) printf("-1\n");
        else printf("%d\n",getans(x,y));
    }
    return 0;
}