(lca)牛客小白月赛13(F 小A的最短路)

#include<iostream>
#include<cstring>
#include<cstdio>
#include<queue>
#include<cstdlib>
#include<cmath>
#include<stack>
#include<map>
#include<vector>
#include<algorithm>
using namespace std;
#define ll long long
#define lb long double
#define INF 0x3f3f3f3f
const int maxn = 300005;
const ll mod = 1e9 + 7;
/*
4
1 2
1 3
2 4
3 4
2
1 3
3 4
*/
int n, q, cnt;
int head[maxn], vis[maxn], lg[maxn], fa[maxn][64], dep[maxn];
struct node{
	int to;
	int next;
	int quan;
}edge[maxn << 1];
void add(int u, int v){
	edge[cnt].to = v;
	edge[cnt].next = head[u];
	head[u] = cnt ++;
}
int lca(int x, int y){
	int ans = 0;
	if(dep[x] < dep[y]) swap(x, y);
	while(dep[x] > dep[y]) x = fa[x][lg[dep[x] - dep[y]] - 1];
	if(x == y) return x;
	for(int i = lg[dep[x]] - 1 ; i >= 0 ; -- i){
		if(fa[x][i] != fa[y][i]){
			x = fa[x][i]; y = fa[y][i];
		}
	}
	return fa[x][0];
}
void dfs(int s, int f){
	dep[s] = dep[f] + 1;
	fa[s][0] = f;
	for(int i = 1 ; (1 << i) <= dep[s] ; ++ i){
		fa[s][i] = fa[fa[s][i-1]][i-1];
	}
	for(int i = head[s] ; i != -1 ; i = edge[i].next){
		int tt = edge[i].to;
		if(!vis[tt]){
			vis[tt] = 1;
			dfs(tt, s);
		}
	}
}
int dist(int x, int y)
{
    int lc = lca(x, y);
    return dep[x] + dep[y] - 2 * dep[lc];
}
int main()
{
	int u, v, x, y;
	scanf("%d", &n);
	memset(head, -1, sizeof(head));
	for(int i = 1 ; i < n ; ++ i){
		scanf("%d %d", &u, &v);
		add(u, v); add(v, u);
	}
	scanf("%d %d", &u, &v);
	for(int i = 1 ; i <= n ; ++ i){
		lg[i] = lg[i-1] + (1 << lg[i-1] == i);
	}
	vis[1] = 1;
	dfs(1, 0);
	scanf("%d", &q);
	while(q--){
		scanf("%d %d", &x, &y);
		printf("%d\n", min(min(dist(x,u)+dist(y,v), dist(x,v)+dist(y,u)), dist(x,y)));
	}
	return 0;
}