洛谷P3379 【模板】最近公共祖先（LCA）（树链剖分）

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#include<iostream>
#include<cstdio>
#include<cstring>
using namespace std;
const int MAXN=2*1e6+10;
#define ls k<<1
#define rs k<<1|1
inline char nc()
{
    static char buf[MAXN],*p1=buf,*p2=buf;
    return p1==p2&&(p2=(p1=buf)+fread(buf,1,MAXN,stdin),p1==p2)?EOF:*p1++;
}
inline int read()
{
    char c=nc();int x=0,f=1;
    while(c<'0'||c>'9'){if(c=='-')f=-1;c=nc();}
    while(c>='0'&&c<='9'){x=x*10+c-'0',c=nc();}
    return x*f;
}
struct node
{
    int u,v,nxt;
}edge[MAXN];
int head[MAXN];
int num=1;
void AddEdge(int x,int y)
{
    edge[num].u=x;
    edge[num].v=y;
    edge[num].nxt=head[x];
    head[x]=num++;
}
int fa[MAXN],deep[MAXN],tot[MAXN],son[MAXN],top[MAXN],cnt;
int dfs1(int now,int f,int dep)
{
    deep[now]=dep;
    fa[now]=f;
    tot[now]=1;
    int maxson=-1;
    for(int i=head[now];i!=-1;i=edge[i].nxt)
    {
        if(edge[i].v==f) continue;
        tot[now]+=dfs1(edge[i].v,now,dep+1);
        if(tot[edge[i].v]>maxson) maxson=tot[edge[i].v],son[now]=edge[i].v;
    }
    return tot[now];
}
void dfs2(int now,int topf)
{
    top[now]=topf;
    if(!son[now]) return ;
    dfs2(son[now],topf);
    for(int i=head[now];i!=-1;i=edge[i].nxt)
        if(edge[i].v!=son[now]&&edge[i].v!=fa[now])
            dfs2(edge[i].v,edge[i].v);
}
int LCA(int x,int y)
{
    while(top[x]!=top[y])
    {
        if(deep[top[x]]<deep[top[y]]) swap(x,y);
        x=fa[top[x]];
    }
    if(deep[x]>deep[y]) swap(x,y);
    return x;
}
int main()
{
    #ifdef WIN32
    freopen("a.in","r",stdin);
    #else
    #endif
    memset(head,-1,sizeof(head));
    int N=read(),M=read(),root=read();
    for(int i=1;i<=N-1;i++) 
    {
        int x=read(),y=read();
        AddEdge(x,y);AddEdge(y,x);
    }
    dfs1(root,0,1);
    dfs2(root,root);
    while(M--)
    {
        int x=read(),y=read();
        printf("%d\n",LCA(x,y));
    }
    return 0;
}