鍊嶅LCA妯℃澘

Posted 2022-04-28

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//https://www.luogu.org/problemnew/show/P3379
#include<bits/stdc++.h>
#define maxn 500010
#define maxm 1000010
using namespace std;

struct edge
    int to,next;
e[maxm];

int point[maxn],n,m,rt,cnt;
int fa[maxn][20],depth[maxn];

void addedge(int x,int y)

    e[++cnt].next=point[x];
    e[cnt].to=y;
    point[x]=cnt;


void dfs(int x,int from)

    depth[x]=depth[from]+1;
    fa[x][0]=from;
    for(int i=point[x];i;i=e[i].next)
    
        int y=e[i].to;
        if(y==from)
            continue;
        dfs(y,x);
    


int go_up(int x,int d)

    for(int j=19;j>=0;j--)
        if((1<<j)&d)
            x=fa[x][j];
    return x;



int main()

    scanf("%d%d%d",&n,&m,&rt);
    for(int i=1;i<=n-1;i++)
    
        int x,y;
        scanf("%d%d",&x,&y);
        addedge(x,y);
        addedge(y,x);
    
    dfs(rt,rt);
    for(int j=1;j<=19;j++)
        for(int i=1;i<=n;i++)
            fa[i][j]=fa[fa[i][j-1]][j-1];
    for(int i=1;i<=m;i++)
    
        int x,y;
        scanf("%d%d",&x,&y);
        if(depth[x]>depth[y])
            x=go_up(x,depth[x]-depth[y]);
        if(depth[y]>depth[x])
            y=go_up(y,depth[y]-depth[x]);
        for(int j=19;j>=0;j--)
            if(fa[x][j]!=fa[y][j])
                x=fa[x][j],y=fa[y][j];
        if(x!=y)
            x=fa[x][0];
        printf("%d\n",x); 
     
    return 0;

#include<bits/stdc++.h>
#define maxn 500010
#define maxm 1000010
using namespace std;

struct edge
    int to,next;
e[maxm];

int point[maxn],n,m,rt,cnt;
int fa[maxn][20],depth[maxn];

void addedge(int x,int y)

    e[++cnt].next=point[x];
    e[cnt].to=y;
    point[x]=cnt;


void dfs(int x,int from)

    depth[x]=depth[from]+1;
    fa[x][0]=from;
    for(int i=point[x];i;i=e[i].next)
    
        int y=e[i].to;
        if(y==from)
            continue;
        dfs(y,x);
    


int go_up(int x,int d)

    for(int j=19;j>=0;j--)
        if((1<<j)&d)
            x=fa[x][j];
    return x;



int main()

    scanf("%d%d%d",&n,&m,&rt);
    for(int i=1;i<=n-1;i++)
    
        int x,y;
        scanf("%d%d",&x,&y);
        addedge(x,y);
        addedge(y,x);
    
    dfs(rt,rt);
    for(int j=1;j<=19;j++)
        for(int i=1;i<=n;i++)
            fa[i][j]=fa[fa[i][j-1]][j-1];
    for(int i=1;i<=m;i++)
    
        int x,y;
        scanf("%d%d",&x,&y);
        if(depth[x]>depth[y])
            x=go_up(x,depth[x]-depth[y]);
        if(depth[y]>depth[x])
            y=go_up(y,depth[y]-depth[x]);
        for(int j=19;j>=0;j--)
            if(fa[x][j]!=fa[y][j])
                x=fa[x][j],y=fa[y][j];
        if(x!=y)
            x=fa[x][0];
        printf("%d\n",x); 
     
    return 0;