树链剖分求LCA

#include<iostream>
#include<cstdio>
#include<cstring>
#include<cstdio>
#include<algorithm>
using namespace std;
#define maxn 1000010
#define IL inline
IL void read(int &x)
{
    x=0;int f=1;char ch=getchar();
    while(ch<'0'||ch>'9'){if(ch=='-')f=-f;ch=getchar();}
    while(ch>='0'&&ch<='9'){x=x*10+ch-'0',ch=getchar();}x*=f;
}

int n,m,s,tot;
int first[maxn],deep[maxn],fa[maxn],son[maxn],size[maxn],top[maxn];
struct edge{
    int nextx,to;
}e[maxn];

void add(int u,int v)
{
    tot++;
    e[tot].nextx=first[u];
    first[u]=tot;
    e[tot].to=v;
}

void dfs1(int x,int f,int depth)
{
    fa[x]=f;
    deep[x]=depth;
    size[x]=1;
    int mson=-1;
    for(int i=first[x];i;i=e[i].nextx)
    {
        int u=e[i].to;
        if(u==f)continue;
        dfs1(u,x,depth+1);
        size[x]+=size[u];
        if(size[u]>mson)
            mson=size[u],son[x]=u;
    }
}

void dfs2(int x,int topf)
{
    top[x]=topf;
    if(!son[x])return;
    dfs2(son[x],topf);
    for(int i=first[x];i;i=e[i].nextx)
    {
        int u=e[i].to;
        if(u==fa[x]||u==son[x])continue;
        dfs2(u,u);
    }
}

int main()
{
    read(n),read(m),read(s);
    for(int i=1;i<=n-1;i++)
    {
        int a,b;
        read(a),read(b);
        add(a,b),add(b,a);
    }
    dfs1(s,s,1);
    dfs2(s,s);
    for(int i=1;i<=m;i++)
    {
        int x,y;
        read(x),read(y);
        while(top[x]!=top[y])
        {
            if(deep[top[x]]>=deep[top[y]])x=fa[top[x]];
            else y=fa[top[y]];
        }
        if(deep[x]<=deep[y])cout<<x<<"
";
        else cout<<y<<"
";
    }
    return 0;
}