【luogu1967】【noip2013】 货车运输 [生成树kruskal LCA ]

#include<iostream>
#include<cstdio>
#include<queue>
#include<cstring>
#include<cmath>
#include<stack>
#include<algorithm>
using namespace std;
#define ll long long
#define rg register
const int N=100000+5,M=500000+5,inf=0x3f3f3f3f,P=19650827;
int n,m,q;
int dep[N],p[N][25],w[N][25];
template <class t>void rd(t &x){
    x=0;int w=0;char ch=0;
    while(!isdigit(ch)) w|=ch==‘-‘,ch=getchar();
    while(isdigit(ch)) x=(x<<1)+(x<<3)+(ch^48),ch=getchar();
    x=w?-x:x;
}

int head[N],tot=0,ans;
struct edge{int v,nxt,w;}e[M];
void add(int u,int v,int w){
    e[++tot]=(edge){v,head[u],w};head[u]=tot;
}

int f[N];
struct node{
    int u,v,w;
}nd[M];
bool cmp(node a,node b){return a.w>b.w;}
int find(int x){return f[x]==x?x:f[x]=find(f[x]);}
void kruskal(){
    int cnt=0;
    for(rg int i=1;i<=n;++i) f[i]=i;
    for(rg int i=1,u,v,w;i<=m;++i){
        u=nd[i].u,v=nd[i].v,w=nd[i].w;
        if(find(u)!=find(v)){
            f[f[u]]=f[v];
            ++cnt;
            add(u,v,w),add(v,u,w);
            if(cnt==n-1) break;
        }
    }
}

void dfs(int u){
    for(int i=head[u],v;i;i=e[i].nxt){
        v=e[i].v;
        if(!dep[v]){//未走过 
            dep[v]=dep[u]+1;
            p[v][0]=u;
            w[v][0]=e[i].w;
            dfs(v);
        }
    }
}
void build(){
    for(rg int i=1;i<=n;++i){
        if(!dep[i]){
            dep[i]=1;
            p[i][0]=0;
            dfs(i);
        }
    }
    dfs(1);//连起来
    for(int i=1; i<=20; i++)
        for(int j=1; j<=n; j++){
            p[j][i]=p[p[j][i-1]][i-1]; 
            w[j][i]=min(w[j][i-1], w[p[j][i-1]][i-1]);
        }
}

int LCA(int a,int b){
    ans=inf;
    if(dep[a]>dep[b]) swap(a,b);
    for(int i=20;i>=0;--i){
        if(dep[p[b][i]]<dep[a]) continue;
        ans=min(ans,w[b][i]);b=p[b][i];
    }
    if(a==b) return ans;
    for(int i=20;i>=0;--i){
        if(p[a][i]==p[b][i]) continue;
        ans=min(ans,min(w[a][i],w[b][i]));
        a=p[a][i],b=p[b][i];
    }
    ans=min(ans,min(w[a][0],w[b][0]));
    return ans;
}

int main(){
    //freopen("in.txt","r",stdin);
    memset(dep,0,sizeof(dep));
    memset(p,0,sizeof(p));
    rd(n),rd(m);
        int u,v,w;
    for(rg int i=1;i<=m;++i){
        rd(u),rd(v),rd(w);
        nd[i]=(node){u,v,w};
    }
    sort(nd+1,nd+1+m,cmp);
    kruskal();
    build();
    rd(q);
    for(rg int i=1;i<=q;++i){
        rd(u),rd(v);
        if(find(u)!=find(v)) printf("-1\n");
        else printf("%d\n",LCA(u,v));
    }
    return 0;
}