bzoj1602 [Usaco2008 Oct]牧场行走

dis[s]+dis[t]-2*dis[lca(s,t)];

#include<bits/stdc++.h>
#define N 2000
using namespace std;
int n,q,x,y,z;
int first[N+5],nxt[N+5],to[N+5],val[N+5],siz;
int f[N+5][25],dis[N+5],dep[N+5];
inline char nc()
{
static char buf[100000],*p1=buf,*p2=buf;
return p1==p2&&(p2=(p1=buf)+fread(buf,1,100000,stdin),p1==p2)?EOF:*p1++;
}
inline int read()
{
int x=0,b=1;
char c=nc();
for(;!(c<='9'&&c>='0');c=nc())if(c=='-')b=-1;
for(;c<='9'&&c>='0';c=nc())x=x*10+c-'0';

4000
return x*b;
}
inline void write(int x)
{
if(x==0)putchar('0');
else
{
char buf[15];
int len=0;
if(x<0)putchar('-'),x=-x;
while(x)buf[++len]=x%10+'0',x/=10;
for(int i=len;i>=1;i--)putchar(buf[i]);
}
putchar('\n');
}
inline void add(int x,int y,int z)
{
nxt[siz]=first[x];
first[x]=siz;
to[siz]=y;
val[siz]=z;
siz++;
}
inline void init(int x,int fa)
{
f[x][0]=fa;
for(int i=1;i<=16;i++)
if(f[x][i-1])f[x][i]=f[f[x][i-1]][i-1];else break;
for(int i=first[x];i!=-1;i=nxt[i])
{
int u=to[i];
if(u==fa)continue;
dep[u]=dep[x]+1;
dis[u]=dis[x]+val[i];
init(u,x);
}
}
inline int LCA(int x,int y)
{
if(dep[x]<dep[y])swap(x,y);
int fet=dep[x]-dep[y];
for(int i=16;i>=0;i--)if(fet>=(1<<i))x=f[x][i],fet-=(1<<i);
if(x==y)return x;
for(int i=16;i>=0;i--)if(f[x][i]!=f[y][i])
x=f[x][i],y=f[y][i];
return f[x][0];
}
int main()
{
freopen("in.txt","r",stdin);
n=read(),q=read();
memset(first,-1,sizeof(first));
for(int i=1;i<n;i++)x=read(),y=read(),z=read(),add(x,y,z),add(y,x,z);
init(1,0);
for(int i=1;i<=q;i++)
x=read(),y=read(),write(dis[x]+dis[y]-2*dis[LCA(x,y)]);
return 0;
}