洛谷-P3379 【模板】最近公共祖先（LCA）（树链剖分）

题目链接：https://www.luogu.org/problemnew/show/P3379

题目大意：LCA.

思路：树链剖分，左右互相找即可。

ACCode：

//#pragma comment(linker, "/STACK:1024000000,1024000000")
  
#include<stdio.h>
#include<string.h> 
#include<math.h> 
   
#include<map>  
#include<set>
#include<deque> 
#include<queue> 
#include<stack> 
#include<bitset>
#include<string> 
#include<fstream>
#include<iostream> 
#include<algorithm> 
using namespace std; 
  
#define ll long long 
#define Pair pair<int,int>
//#define max(a,b) (a)>(b)?(a):(b)
//#define min(a,b) (a)<(b)?(a):(b)
#define clean(a,b) memset(a,b,sizeof(a))// ??
//std::ios::sync_with_stdio(false);
//  register
const int MAXN=5e5+10;
const int INF32=0x3f3f3f3f;
const ll INF64=0x3f3f3f3f3f3f3f3f;
const ll mod=1e9+7;
const double PI=acos(-1.0);
const double EPS=1.0e-8;

class Segment{
	int P;
	int Tree[MAXN<<2],Lazy[MAXN<<2],Size[MAXN<<2];
	
	void PushDown(int rt){
		if(Lazy[rt]==0) return ;
		Tree[rt<<1]=(Tree[rt<<1]+Size[rt<<1]*Lazy[rt]+P)%P;
		Tree[rt<<1|1]=(Tree[rt<<1|1]+Size[rt<<1|1]*Lazy[rt]+P)%P;
		
		Lazy[rt<<1]=(Lazy[rt<<1]+Lazy[rt]+P)%P;
		Lazy[rt<<1|1]=(Lazy[rt<<1|1]+Lazy[rt]+P)%P;
		Lazy[rt]=0;
	}
	public : void SetMod(int Mod){
		P=Mod;
	}
	public : void Build(int l,int r,int rt,int a[]){
		Size[rt]=r-l+1;
//		printf("rt=%d l=%d r=%d Tree[rt]=%d\n",rt,l,r,Tree[rt]);
		if(l==r){
			Tree[rt]=a[l];
			return ;
		}int mid=(l+r)>>1;
		Build(l,mid,rt<<1,a);Build(mid+1,r,rt<<1|1,a);
		Tree[rt]=(Tree[rt<<1]+Tree[rt<<1|1]+P)%P;
	}
	public : void Update(int ql,int qr,int val,int l,int r,int rt){
		if(ql<=l&&r<=qr){
			Tree[rt]+=Size[rt]*val;
			Lazy[rt]+=val;
			return ;
		}int mid=(l+r)>>1;
		PushDown(rt);
		if(ql<=mid) Update(ql,qr,val,l,mid,rt<<1);
		if(qr>mid) Update(ql,qr,val,mid+1,r,rt<<1|1);
		Tree[rt]=(Tree[rt<<1]+Tree[rt<<1|1]+P)%P;
	}
	public : int Query(int ql,int qr,int l,int r,int rt){
		if(ql<=l&&r<=qr) return Tree[rt];
		PushDown(rt);
		int mid=(l+r)>>1;
		int ans=0;
		if(ql<=mid) ans=(ans+Query(ql,qr,l,mid,rt<<1)+P)%P;
		if(qr>mid) ans=(ans+Query(ql,qr,mid+1,r,rt<<1|1)+P)%P;
		return ans;
	}
	public : void Show(int l,int r,int rt){
		printf("rt=%d l=%d r=%d Tree[rt]=%d\n",rt,l,r,Tree[rt]);
		if(l==r) return ;
		int mid=(l+r)>>1;
		Show(l,mid,rt<<1);Show(mid+1,r,rt<<1|1);
	}
};
struct Node1{
	int v,val,nxt;
	Node1(int _v=0,int _val=0,int _nxt=0){
		v=_v;val=_val;nxt=_nxt;
	}
};
//Segment Seg;
Node1 Edge[MAXN<<2];
int Head[MAXN],Ecnt;
int Deep[MAXN],Fa[MAXN],Size[MAXN],Son[MAXN];
int Idx[MAXN],Icnt;//重新标号 
int Top[MAXN];
int A[MAXN],B[MAXN];
int n,m,R;

void Intt(){
	clean(Head,-1);Ecnt=0;
	clean(Deep,0);clean(Fa,-1);clean(Size,0);clean(Son,-1);
	Icnt=0;
}
void AddEdge(int u,int v,int val){
	Edge[Ecnt]=Node1(v,val,Head[u]);
	Head[u]=Ecnt++;
}
int DFS1(int u,int fa,int dep){
	Deep[u]=dep;
	Fa[u]=fa;
	Size[u]=1;
	int maxson=-1;
	for(int i=Head[u];i+1;i=Edge[i].nxt){
		int temp=Edge[i].v;
		if(temp==fa) continue;
		Size[u]+=DFS1(temp,u,dep+1);
		if(Size[temp]>maxson){
			maxson=Size[temp];
			Son[u]=temp;
		}
	}return Size[u];
}
void DFS2(int u,int topfa){
	Idx[u]=++Icnt;
	Top[u]=topfa;
	if(Son[u]==-1) return ;
	DFS2(Son[u],topfa);
	for(int i=Head[u];i+1;i=Edge[i].nxt){
		int temp=Edge[i].v;
		if(Idx[temp]==0){
			DFS2(temp,temp);
		}
	}
}
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(){
	scanf("%d%d%d",&n,&m,&R);
	Intt();
	for(int i=1;i<n;++i){
		int x,y;scanf("%d%d",&x,&y);
		AddEdge(x,y,1);AddEdge(y,x,1);
	}
	DFS1(R,-1,1);
	DFS2(R,R);
	for(int i=1;i<=m;++i){
		int x,y;scanf("%d%d",&x,&y);
		printf("%d\n",LCA(x,y));
	}
}