货车运输 LCA
陈子骏
2018-04-02 17:49:21
```
// LCA
#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstring>
#define inf 0x7fffffff
using namespace std;
int n,m,q,cnt,tot;
int deep[10001],head[10001],f[10001],fa[10001][17],d[10001][17];
bool vis[10001];
struct edge{
int x,y,v;
}a[50001];
struct e{
int next,to,v;
}e[20001];
void ins(int u,int v,int w)
{
e[++cnt].to=v;
e[cnt].next=head[u];
head[u]=cnt;
e[cnt].v=w;
}
void insert(int u,int v,int w){
ins(u,v,w);
ins(v,u,w);
}
int find(int x){return f[x]==x?x:f[x]=find(f[x]);}
bool cmp(edge a,edge b){return a.v>b.v;}
void dfs(int x){
vis[x]=1;
for(int i=1;i<=16;i++){
if(deep[x]<(1<<i))break;
fa[x][i]=fa[fa[x][i-1]][i-1];
d[x][i]=min(d[x][i-1],d[fa[x][i-1]][i-1]);
}
for(int i=head[x];i;i=e[i].next){
if(vis[e[i].to])continue;
fa[e[i].to][0]=x;
d[e[i].to][0]=e[i].v;
deep[e[i].to]=deep[x]+1;
dfs(e[i].to);
}
}
int lca(int x,int y){
if(deep[x]<deep[y])swap(x,y);
int t=deep[x]-deep[y];
for(int i=0;i<=16;i++)
if((1<<i)&t)x=fa[x][i];
for(int i=16;i>=0;i--){
if(fa[x][i]!=fa[y][i]){
x=fa[x][i];
y=fa[y][i];
}
}
if(x==y)return x;
return fa[x][0];
}
int ask(int x,int f){
int mn=inf;
int t=deep[x]-deep[f];
for(int i=0;i<=16;i++){
if(t&(1<<i)){
mn=min(mn,d[x][i]);
x=fa[x][i];
}
}
return mn;
}
int main(){
memset(d,127/3,sizeof(d));
scanf("%d%d",&n,&m);
for(int i=1;i<=n;i++)f[i]=i;
for(int i=1;i<=m;i++)
scanf("%d%d%d",&a[i].x,&a[i].y,&a[i].v);
sort(a+1,a+m+1,cmp);
for(int i=1;i<=m;i++){
int x=a[i].x,y=a[i].y;
int p=find(a[i].x),q=find(a[i].y);
if(p!=q){
f[p]=q;
insert(x,y,a[i].v);
tot++;if(tot==n-1)break;
}
}
for(int i=1;i<=n;i++)if(!vis[i])dfs(i);
scanf("%d",&q);
for(int i=1;i<=q;i++){
int x,y;scanf("%d%d",&x,&y);
if(find(x)!=find(y)){
printf("-1\n");
continue;
}else{
int t=lca(x,y);
printf("%d\n",min(ask(x,t),ask(y,t)));
}
}
return 0;
}
```