宁波多校（二） D. LCA！（换根lca）

#include<iostream>
#include<queue>
#include<map>
#include<vector>
#include<cstdio>
#include<algorithm>
#include<stack>
#include<cstring>
using namespace std;
typedef long long ll;
const int N=2e5+10;
int h[N],ne[N],e[N],idx;
int n,q;
int depth[N],fa[N][25];
int sz[N];
int dfn[N];
int times;
void add(int a,int b){
    e[idx]=b,ne[idx]=h[a],h[a]=idx++;
}
void bfs(){
    memset(depth,0x3f,sizeof depth);
    depth[0]=0,depth[1]=1;
    int i;
    queue<int> q;
    q.push(1);
    while(q.size()){
        int t=q.front();
        q.pop();
        for(i=h[t];i!=-1;i=ne[i]){
            int j=e[i];
            if(depth[j]>depth[t]+1){
                depth[j]=depth[t]+1;
                q.push(j);
                fa[j][0]=t;
                for(int k=1;k<=21;k++){
                    fa[j][k]=fa[fa[j][k-1]][k-1];
                }
            }
        }
    }
}
int lca(int a,int b){
    if(depth[a]<depth[b])
        swap(a,b);
    int i;
    for(i=21;i>=0;i--){
        if(depth[fa[a][i]]>=depth[b]){
            a=fa[a][i];
        }
    }
    if(a==b)
        return a;
    for(i=21;i>=0;i--){
        if(fa[a][i]!=fa[b][i]){
            a=fa[a][i];
            b=fa[b][i];
        }
    }
    return fa[a][0];
}
int dfs(int u,int fa){
    dfn[u]=++times;
    sz[u]=1;
    int i;
    for(i=h[u];i!=-1;i=ne[i]){
        int j=e[i];
        if(j==fa)
            continue;
        dfs(j,u);
        sz[u]+=sz[j];
    }
    return sz[u];
}
int main(){
    int n;
    while(cin>>n){
        int i;
        memset(h,-1,sizeof h);
        for(i=1;i<n;i++){
            int a,b;
            scanf("%d%d",&a,&b);
            add(a,b);
            add(b,a);
        }
        bfs();
        int m;
        cin>>m;
        dfs(1,-1);
        while(m--){
            int root,x,y;
            scanf("%d%d%d",&root,&x,&y);
            if(dfn[x]>dfn[y])
                swap(x,y);
            int p=lca(x,y);
            int ans=p;
            if(root==1)
                ans=p;
            else if(lca(root,p)==root){
                ans=p;
            }
            else if(lca(root,x)==p&&lca(root,y)==p){
                ans=p;
            }
            else{
                if(lca(x,y)==x){
                    if(lca(root,x)==root)
                        ans=x;
                    else if(lca(root,x)==x&&lca(root,y)==root){
                        ans=root;
                    }
                    else if(lca(root,x)==x&&lca(root,y)!=root){
                        ans=lca(root,y);
                    }
                    else if(lca(root,x)==x&&lca(root,y)==y)
                        ans=y;
                }
                else{
                    if(lca(root,x)==root&&lca(root,y)==p)
                        ans=root;
                    else if(lca(root,x)==p&&lca(root,y)==root)
                        ans=root;
                    else if(lca(root,x)==x)
                        ans=x;
                    else if(lca(root,y)==y)
                        ans=y;
                    else if(lca(root,x)==p&&lca(root,y)!=root)
                        ans=lca(root,y);
                    else if(lca(root,x)!=root&&lca(root,y)==p)
                        ans=lca(root,x);
                }
            }
            cout<<ans<<endl;
        }
    }

}