题意：

思路：

#include <cstdio>
#include <algorithm>
#include <cstring>
using namespace std;
const int N = 3e5 + 10;
struct Edge{
    int to,nex;
}e[N << 1];
int head[N],dep[N],fa[N][20],idx,n,A,B;
void add_edge(int u,int v){
    e[idx].to = v;
    e[idx].nex = head[u];
    head[u] = idx++;
}
void dfs(int u,int f){
    dep[u] = dep[f] + 1;
    fa[u][0] = f;
    for(int i = head[u];~i;i = e[i].nex){
        int v = e[i].to;
        if(v == f) continue;
        dfs(v,u);
    }
}
void init(){
    for(int k = 1;(1<<k) <= n;k++){
        for(int i = 1;i <= n;i++){
            fa[i][k] = fa[fa[i][k-1]][k-1];
        }
    }
}
int lca(int u,int v){
    if(dep[u] < dep[v]) swap(u,v);
    if(dep[u] != dep[v]){
        for(int i = 19;i >= 0;i--){
            if(dep[fa[u][i]] >= dep[v]) u = fa[u][i];
        }
    }
    if(u == v) return v;
    for(int i = 19;i >= 0;i--){
        if(fa[u][i] != fa[v][i]){
            u = fa[u][i],v = fa[v][i];
        }
    }
    return fa[u][0];
}
int dist(int u,int v){
    return dep[u] + dep[v] - (dep[lca(u,v)] * 2);
}
int main(){
    memset(head,-1,sizeof(head));
    scanf("%d",&n);
    for(int i = 1,u,v;i < n;i++){
        scanf("%d%d",&u,&v);
        add_edge(u,v);add_edge(v,u); 
    }    
    dfs(1,0);
    init();
    scanf("%d%d",&A,&B);
    int q;scanf("%d",&q);
    while(q--){
        int u,v;scanf("%d%d",&u,&v);
        int res = dist(u,v);
        res = min(res,min(dist(u,B) + dist(A,v),dist(u,A) + dist(B,v)));
        printf("%d\n",res);
    }
    return 0;
}