1832: [AHOI2008]聚会 LCA

每次取三个LCA讨论。。

#include<iostream>
#include<cstdio>
#define N 500005
#define inf 1000000007
using namespace std;
int n,m,cnt;
int head[N],deep[N],fa[N][20];
int next[N<<1],list[N<<1];
inline int read()
{
    int a=0,f=1; char c=getchar();
    while (c<'0'||c>'9') {if (c=='-') f=-1; c=getchar();}
    while (c>='0'&&c<='9') {a=a*10+c-'0'; c=getchar();}
    return a*f;
}
inline void insert(int x,int y)
{
    next[++cnt]=head[x];
    head[x]=cnt;
    list[cnt]=y;
}
void dfs(int x)
{
    for (int i=1;(1<<i)<=deep[x];i++)
        fa[x][i]=fa[fa[x][i-1]][i-1];
    for (int i=head[x];i;i=next[i])
        if (list[i]!=fa[x][0])
        {
            fa[list[i]][0]=x;
            deep[list[i]]=deep[x]+1;
            dfs(list[i]);
        }
}
inline int lca(int x,int y)
{
    if (deep[x]<deep[y]) swap(x,y);
    int t=deep[x]-deep[y];
    for (int i=0;(1<<i)<=t;i++) 
        if (t&(1<<i)) x=fa[x][i];
    for (int i=19;~i;i--)
        if (fa[x][i]!=fa[y][i]) x=fa[x][i],y=fa[y][i];
    return x==y?x:fa[x][0];
}
inline int calc(int x,int y,int f)
{
    return deep[x]+deep[y]-2*deep[f];   
}
inline int calc0(int x,int y)
{
    int t=lca(x,y);
    return calc(x,y,t);
}
int main()
{
    n=read(); m=read();
    for (int i=1;i<n;i++) 
    {
        int u=read(),v=read();
        insert(u,v); insert(v,u);
    }
    dfs(1);
    for (int i=1;i<=m;i++)
    {
        int x=read(),y=read(),z=read();
        int t1=lca(x,y),t2=lca(y,z),t3=lca(x,z);
        int dis1=calc(x,y,t1)+calc0(z,t1);
        int dis2=calc(y,z,t2)+calc0(x,t2);
        int dis3=calc(x,z,t3)+calc0(y,t3);
        int mx=inf,k=0;
        if (dis1<mx) mx=dis1,k=1;
        if (dis2<mx) mx=dis2,k=2;
        if (dis3<mx) mx=dis3,k=3;
        if (k==1) printf("%d %d\n",t1,dis1);
        if (k==2) printf("%d %d\n",t2,dis2);
        if (k==3) printf("%d %d\n",t3,dis3);
    }
    return 0;
}