lca tle*3 敢问dalao如何优化

#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
const int maxn=500001;
int n,m,a,b,head[maxn],deep[maxn],p[maxn][31/*大概*/],num=0,vis[maxn];
struct EDGE{
    int next,to;
}edge[2*maxn];
int read()
{
    int x=0;char ch=getchar();
    while(ch<'0'||ch>'9')ch=getchar();
    while(ch>='0'&&ch<='9')x=x*10+ch-'0',ch=getchar();
    return x;
}
void add(int from,int to){
    edge[++num].next=head[from];
    edge[num].to=to;
    head[from]=num;
}
void dfs(int x){
    for(int i=head[x];i;i=edge[i].next){
        if(vis[edge[i].to]!=1){
            vis[edge[i].to]=1;
            deep[edge[i].to]=deep[x]+1;
            p[edge[i].to][0]=x;
            dfs(edge[i].to);
        }
    }
}
int lca(int a,int b){
    int i,j;
    if(deep[a]<deep[b]){
        int t=a;a=b;b=t;
    }
    for(i=0;(1<<i)<=deep[a];i++);
    i--;
    for(j=i;j>=0;j--){
        if(deep[a]-(1<<j)>=deep[b]){
            a=p[a][j];
        }
    }
    if(a==b)return a;
    for(j=i;j>=0;j--){
        if(p[a][j]!=p[b][j]&&p[a][j]!=-1/*也许不需要*/){
            a=p[a][j];b=p[b][j];
        }
    }
    return p[a][0];
}
int main(){
    memset(deep,0,sizeof(deep));memset(p,-1,sizeof(p));memset(vis,0,sizeof(vis));
    int d;n=read();m=read();d=read();
    for(int i=1;i<=n-1;i++){
        int u=read(),v=read();
        add(u,v);add(v,u);
    }
    deep[d]=0;vis[d]=1;
    dfs(d);
    for(int j=1;j<=21;j++)
      for(int i=1;i<=n;i++){
          if(p[i][j-1]!=-1){
              p[i][j]=p[p[i][j-1]][j-1];
          }
      }
    while(m--)a=read(),b=read(),cout<<lca(a,b)<<endl;
    return 0;
}