链接Miku

树刨理论

#include<iostream>
#include<cstdio>
#include<algorithm>
#include<cstring>
using namespace std;
const int maxn=500001;
int n,m,s;
int fa[maxn];
int son[maxn];
int siz[maxn];
int dee[maxn];
int top[maxn];
int head[maxn];
int p;
int x,y;
struct e{
    int to;
    int f;
    int ne;
}ed[maxn*2+1];
void add(int f,int t){
    p++;
    ed[p].f=f;
    ed[p].to=t;
    ed[p].ne=head[f];
    head[f]=p;
    return ;
}
void dfs1(int now,int f){
    siz[now]=1;
    fa[now]=f;
    dee[now]=dee[f]+1;
    for(int i=head[now];i;i=ed[i].ne){
        if(ed[i].to==f)
        continue;
        dfs1(ed[i].to,now);
        siz[now]+=siz[ed[i].to];
        if(!son[now]||siz[son[now]]<siz[ed[i].to]){
            son[now]=ed[i].to;
        }
    }
    return ;
}
void dfs2(int now,int f){
    top[now]=f;
    for(int i=head[now];i;i=ed[i].ne){
        if(ed[i].to==fa[now]) continue;
        else if(ed[i].to==son[now]) dfs2(ed[i].to,f);
        else dfs2(ed[i].to,ed[i].to);
    } 
    return ;
}
int main(){
    scanf("%d%d%d",&n,&m,&s);
    for(int i=1;i<n;++i){
        scanf("%d%d",&x,&y);
        add(x,y);
        add(y,x);
    }
    dfs1(s,s); 
    dfs2(s,s);
    for(int i=1;i<=m;++i){
        scanf("%d%d",&x,&y);
        while(top[x]!=top[y]){
            dee[top[x]]>=dee[top[y]] ? x=fa[top[x]] :y=fa[top[y]];
        }
        x= dee[x]>dee[y] ? y:x;
        cout<<x<<endl;
    }
    return 0;
}