[Bzoj1602][Usaco2008 Oct]牧场行走

#include<iostream>
#include<cstdio>
using namespace std;
int n,q,cnt,fa[1001][11],deep[1001],head[1001],dis[1001];
bool vis[1001];
struct data{
int to,next,v;
}e[1001];
void ins(int u,int v,int w){
e[++cnt].to=v;
e[cnt].v=w;
e[cnt].next=head[u];
head[u]=cnt;
}
void insert(int u,int v,int w){
ins(u,v,w);ins(v,u,w);
}
void dfs(int x){
vis[x]=true;
for(int i=1;i<=9;i++){
if(deep[x]<(1<<i))break;
fa[x][i]=fa[fa[x][i-1]][i-1];
}
for(int i=head[x];i;i=e[i].next){
if(vis[e[i].to])continue;
deep[e[i].to]=deep[x]+1;
dis[e[i].to]=dis[x]+e[i].v;
fa[e[i].to][0]=x;
dfs(e[i].to);
}
}
int lca(int x,int y){
if(deep[x]<deep[y])swap(x,y);
int d=deep[x]-deep[y];
for(int i=0;i<=9;i++)
if((1<<i)&d)x=fa[x][i];
for(int i=9;i>=0;i--)
if(fa[x][i]!=fa[y][i])
x=fa[x][i],y=fa[y][i];
if(x==y)return x;
else return fa[x][0];
}
int main(){
scanf("%d%d",&n,&q);
for(int i=1;i<n;i++){
int a,b,l;
scanf("%d%d%d",&a,&b,&l);
insert(a,b,l);
}dfs(1);
for(int i=1;i<=q;i++){
int a,b;
scanf("%d%d",&a,&b);
printf("%d\n",dis[a]+dis[b]-2*dis[lca(a,b)]);
}
return 0;
}