临时错板子-526互联

#include<bits/stdc++.h>
#define endl '\n'
#define int long long 
using namespace std;
const int N=5e5+1;
int n,m,s,a,b;
int fa[N],son[N],dep[N],top[N],sz[N];
vector<int>e[N];
template<typename Tp> inline void read(Tp&x)
{
    x=0;register bool z=1;
    register char c=getchar();
    for(;c<'0'||c>'9';c=getchar()) if(c=='-') z=0;
    for(;'0'<=c&&c<='9';c=getchar()) x=(x<<1)+(x<<3)+(c^48);
    x=(z?x:~x+1);
}
void dfs1(int x,int father)
{
    fa[x]=father,dep[x]=dep[father]+1,sz[x]=1;
    for(int y:e[x])
        if(y!=father)
        {
            dfs1(y,x);
            sz[x]+=sz[y];
            if(sz[son[x]]<sz[y]) son[x]=y;
        } 
}
void dfs2(int x,int t)
{
    top[x]=t;
    if(!son[x]) return ;
    dfs2(son[x],t);
    for(int y:e[x])
        if(y!=fa[x]&&y!=son[x])
            dfs2(y,y);
}
int lca(int x,int y)
{
    while(top[x]!=top[y])
    {
        if(dep[top[x]]<dep[top[y]]) swap(x,y);
        x=fa[son[x]];
    }
    return dep[x]<dep[y]?x:y;
}
signed main()
{
    #ifndef ONLINE_JUDGE
    freopen("in.txt","r",stdin);
    freopen("out.txt","w",stdout);
    #endif
    read(n),read(m),read(s);
    for(int i=1;i<n;i++)
        read(a),read(b),
        e[a].push_back(b),
        e[b].push_back(a);
    dfs1(s,0),dfs2(s,s);
    while(m--)
        read(a),read(b),
        cout<<lca(a,b)<<endl;
}