[bzoj 1602] [Usaco2008 Oct]牧场行走

求树上两点的距离

裸LCA 用了倍增

#include<cstdio>
#include<cstring>
#include<cmath>
#include<algorithm>
using namespace std;
const int maxn=1233;
int n,q,cnt;
int last[maxn],deep[maxn],dist[maxn];
int f[maxn][20];
bool vis[maxn];
struct graph{
    int to,next,weight;
}e[maxn*2];
void insert(int u,int v,int w){
    e[++cnt].to=v;
    e[cnt].weight=w;
    e[cnt].next=last[u];
    last[u]=cnt;
}
void dfs(int x){
    vis[x]=1;
    for (int i=1;(1<<i)<=deep[x];i++)
        f[x][i]=f[f[x][i-1]][i-1];
    for (int i=last[x];i;i=e[i].next)
        if (!vis[e[i].to]){
            int now=e[i].to;
            deep[now]=deep[x]+1;
            dist[now]=dist[x]+e[i].weight;
            f[now][0]=x;
            dfs(now);
        }
}
int lca(int x,int y){
    if (deep[x]<deep[y]) swap(x,y);
    int d=deep[x]-deep[y];
    for (int i=0;(1<<i)<=d;i++)
       if ((1<<i)&d) x=f[x][i];
    if (x==y) return x;
    for (int i=10;i>=0;i--)
      if (f[x][i]!=f[y][i])
        x=f[x][i],y=f[y][i];
    return f[x][0];
}
int main(){
    scanf("%d%d",&n,&q);
    memset(dist,0,sizeof(dist));
    int u,v,w;
    for (int i=1;i<n;i++){
        scanf("%d%d%d",&u,&v,&w);
        insert(u,v,w),insert(v,u,w);
    }
    dfs(1);
    for (int i=1;i<=q;i++){
        scanf("%d%d",&u,&v);
        printf("%d
",dist[u]+dist[v]-2*dist[lca(u,v)]);
    }
    return 0;
}