[模板] 树链剖分找LCA

#include <cstdio>
#include <cstring>
#define MAX 500005

int d[MAX],fa[MAX],size[MAX],top[MAX],son[MAX];
int N,M,S,tot=0;
int head[MAX];

struct edge{
	int v,next;
}G[MAX<<1];

inline void add(int u,int v){
	G[++tot].v=v;G[tot].next=head[u];head[u]=tot;
}

inline void dfs1(int u,int father){
	d[u]=d[father]+1;
	fa[u]=father;
	size[u]=1;
	int max = 0;
	for(register int i=head[u];i;i=G[i].next){
		int v = G[i].v;
		if(v==father)continue;
		dfs1(v,u);
		size[u]+=size[v];
		if(size[v]>max)son[u]=v,max=size[v];
	}
}

inline void dfs2(int u,int high,int father){
	top[u]=high;
	if(son[u]==0)return;
	dfs2(son[u],high,u);
	for(register int i=head[u];i;i=G[i].next){
		int v = G[i].v;
		if(v==son[u]||v==father)continue;
		dfs2(v,v,u);
	}
}

inline void swap(int* u,int* v){
	int t = *u;*u = *v;*v = t;
}

inline int LCA(int u,int v){
	while(top[u]!=top[v]){
		if(d[top[u]]<d[top[v]])swap(&u,&v);
		u = fa[top[u]];
	}
	return d[u]>d[v]?v:u;
}
int main(){

	std::memset(head,0,sizeof(head));
	std::memset(son,0,sizeof(son));
		
	scanf("%d%d%d",&N,&M,&S);
	int u,v;
	for(register int i=1;i<N;++i){
		scanf("%d%d",&u,&v);
		add(u,v);add(v,u);
	}
	
	d[0]=0;
	
	dfs1(S,0);
	dfs2(S,S,0);
	
	for(register int i=1;i<=M;++i){
		scanf("%d%d",&u,&v);
		printf("%d\n",LCA(u,v));
	}
	return 0;
}