树链剖分求lca

void dfs1(int u,int f)
{
    fa[u]=f,siz[u]=1,dep[u]=dep[f]+1;
    int maxson = -1;
    for(int i=Head[u];~i;i=Edge[i].next)
    {
        int &v = Edge[i].to;
        if(v==f) continue;
        dfs1(v,u);
        siz[u]+=siz[v];
        if(siz[v]>maxson)
            maxson=siz[v],son[u]=v;
    }
}
int top[maxn];
void dfs2(int u,int t)
{
    top[u]=t;
    if(!son[u]) return;
    dfs2(son[u],t);
    for(int i=Head[u];~i;i=Edge[i].next)
    {
        int &v = Edge[i].to;
        if(v==fa[u]||v==son[u]) continue;
        dfs2(v,v);
    }
}
int main()
{
    memset(Head,-1,sizeof(Head));
    int n,m,s;
    read(n,m,s);
    while(--n)
    {
        int u,v;
        read(u,v);
        AddEdge(u,v);
    }
    dfs1(s,s);
    dfs2(s,s);
    while(m--)
    {
        int u,v;
        read(u,v);
        while(top[u]!=top[v])
        {
            if(dep[top[u]]<dep[top[v]]) std::swap(u,v);
            u=fa[top[u]];
        }
        print(dep[u]<dep[v]?u:v);
    }
}