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));
}
}