《洛谷4281》聚会

思路：

显然那个集合点肯定是某两个点的LCA。

因为到LCA,两个点走的距离不会重叠。另外一点到LCA也没有重叠。

那么直接枚举LCA判断就行。

这里时间卡的很紧。

于是：链式向前星存图，倍增常数优化，卡卡常。

Code:

#include<bits/stdc++.h>
using namespace std;
typedef long long LL;
typedef pair<int,int> pii;
const int N = 5e5+5;
const int M = 1e6+5;
const int Mod = 1e9+7;
#define pi acos(-1)
#define INF 1e8
#define INM INT_MIN
#define rg register
#define pb(a)  push_back(a)
#define mk(a,b) make_pair(a,b)
#define dbg(x) cout << "now this num is " << x << endl;
#define met0(axx) memset(axx,0,sizeof(axx));
#define metf(axx) memset(axx,-1,sizeof(axx));
#define sd(ax) scanf("%d",&ax)
#define sld(ax) scanf("%lld",&ax)
#define sldd(ax,bx) scanf("%lld %lld",&ax,&bx)
#define sdd(ax,bx) scanf("%d %d",&ax,&bx)
#define sddd(ax,bx,cx) scanf("%d %d %d",&ax,&bx,&cx)
#define sfd(ax) scanf("%lf",&ax)
#define sfdd(ax,bx) scanf("%lf %lf",&ax,&bx)
#define pr(a) printf("%d\n",a)
#define plr(a) printf("%lld\n",a)
int f[N][25],lg[N],dep[N],cnt = 0,head[N];
struct Node{int to,next;}e[N<<1];
void add(int u,int v)
{
    e[++cnt].to = v,e[cnt].next = head[u],head[u] = cnt;
}
inline int read()
{
    int x = 0,f = 1;char c = getchar();
    while(c < '0' || c > '9'){if(c == '-') f = -1;c = getchar();}
    while(c >= '0' && c <= '9'){x = (x<<1)+(x<<3)+(c^48);c = getchar();}
    return x*f;
}
void init()
{
   for(int i=1;i<N;++i) lg[i] = lg[i-1]+(1<<lg[i-1] == i);
}
void dfs(int u,int fa)
{
    dep[u] = dep[fa]+1,f[u][0] = fa;
    for(rg int i=1;i<=lg[dep[u]];++i) f[u][i] = f[f[u][i-1]][i-1];
    for(rg int i = head[u];i;i=e[i].next) if(e[i].to != fa) dfs(e[i].to,u);
}
int LCA(int x,int y)
{
    if(dep[x] < dep[y]) swap(x,y);
    while(dep[x] > dep[y]) x = f[x][lg[dep[x]-dep[y]]-1];
    if(x == y) return x;
    for(rg int i = lg[dep[x]]-1;i >= 0;--i) 
    {
        if(f[x][i] != f[y][i]) x = f[x][i],y = f[y][i];
    } 
    return f[x][0];
}
int dis(int x,int y)
{
    return dep[x]+dep[y]-2*dep[LCA(x,y)];
}
void slove()
{
    init();
    int n,m;
    n = read(),m = read();
    for(rg int i=1;i<n;++i)
    {
        int u,v;
        u = read(),v = read();
        add(u,v);add(v,u);
    }
    dfs(1,0);
    for(rg int i=1;i<=m;++i)
    {
        int x,y,z;
        x = read(),y = read(),z = read();
        int lca = LCA(x,y);
        int ans = dis(x,y)+dis(lca,z),pos = lca;
        lca = LCA(x,z);
        int tmp = dis(x,z)+dis(lca,y);
        if(tmp < ans) ans = tmp,pos = lca;
        lca = LCA(y,z);
        tmp = dis(y,z)+dis(lca,x);
        if(tmp < ans) ans = tmp,pos = lca;
        printf("%d %d\n",pos,ans);
    }
}  
int main()
{
    slove();
    system("pause");
    return 0;
}