codevs 1503 愚蠢的宠物

1503 愚蠢的宠物

#include<iostream>  
#include<algorithm>  
#include<cstring>  
using namespace std;  
  
#define maxn 1000000+100  
int father[maxn];  
int a[maxn],b[maxn];  
int main()  
{  
    int n,x,y;  
    memset(a,0x3f,sizeof(a));  
    memset(b,0x3f,sizeof(b));  
    cin>>n;  
    for(int i=1;i<=n;i++)  
        father[i]=i;  
    for(int i=1;i<n;i++)  
    {  
        cin>>x>>y;  
        father[y]=x;  
    }  
    cin>>x>>y;  
    for(int i=0;i<n;i++)  
    {  
        a[x]=min(i,a[x]);  
        x=father[x];  
        b[y]=min(i,b[y]);  
        y=father[y];  
    }  
    int mn=999999,ans;  
    for(int i=1;i<=n;i++)  
    {  
        if(a[i]+b[i]<mn){  
            mn=a[i]+b[i];  
            ans=i;  
        }  
    }  
    cout<<ans;  
    return 0;  
} 

#include<bits/stdc++.h>
using namespace std;
#define maxn 1000000
int n,x,y,head[maxn],num;
int deep[maxn],size[maxn],dad[maxn],top[maxn];

struct Edge{
    int v,next;
}edge[maxn];

void add(int u,int v)
{
    edge[++num].v=v;
    edge[num].next=head[u];
    head[u]=num;
}

void dfs(int x)
{
    size[x]=1 ; deep[x]=deep[dad[x]]+1;
    for(int i=head[x];i;i=edge[i].next)
    {
        int v=edge[i].v;
        if(dad[x]==v) continue;
        dad[v]=x; dfs(v); size[x]+=size[v];
     }
}

void dfs2(int x)
{
    int t=0; if(!top[x]) top[x]=x;
    for(int i=head[x];i;i=edge[i].next)
    {
        int v=edge[i].v;
        if(dad[x]!=v&&size[t]<size[v]) t=v;
    }
    if(t) top[t]=top[x],dfs2(t);
    for(int i=head[x];i;i=edge[i].next)
    {
        int v=edge[i].v;
        if(dad[x]!=v&&t!=v) dfs2(v);
    }
}

int lca(int x,int y)
{
    for(;top[x]!=top[y];x=dad[top[x]])
        if(deep[top[x]]<deep[top[y]]) swap(x,y);
    return deep[x]<deep[y]?x:y;
}


int main()
{
    scanf("%d",&n);
    for(int i=1;i<n;i++)
    {
        scanf("%d%d",&x,&y);
        add(x,y); add(y,x);
    }
    dfs(1); dfs2(1);
    int a,b,ans;
    scanf("%d%d",&a,&b);
    printf("%d",lca(a,b));
    return 0;
}