【模板】倍增LCA

题号：洛谷3379

%:pragma GCC optimize ("Ofast")
#include<cstdio>
#include<vector>
#include<cctype>
inline int getint() {
    int ch;
    while(!isdigit(ch=getchar()));
    int x=ch^'0';
    while(isdigit(ch=getchar())) x=(((x<<2)+x)<<1)+(ch^'0');
    return x;
}
const int N=500001,logN=19;
std::vector<int> e[N];
int anc[N][logN+1]={{0}},dep[N];
inline void add_edge(const int u,const int v) {
    e[u].push_back(v);
    e[v].push_back(u);
}
void dfs(const int x,const int parent,const int depth) {
    anc[x][0]=parent,dep[x]=depth;
    for(int i=1;i<=logN;i++) {
        anc[x][i]=anc[anc[x][i-1]][i-1];
    }
    for(unsigned int i=0;i<e[x].size();i++) {
        if(e[x][i]==parent) continue;
        dfs(e[x][i],x,depth+1);
    }
}
inline void swap(int &a,int &b) {
    int t;
    t=a;
    a=b;
    b=t;
}
int LCA(int a,int b) {
    if(dep[a]<dep[b]) swap(a,b);
    for(int i=logN;i>=0;i--) {
        if(dep[a]-(1<<i)>=dep[b]) a=anc[a][i];
    }
    if(a==b) return a;
    for(int i=logN;i>=0;i--) {
        if(anc[a][i]&&anc[a][i]!=anc[b][i]) {
            a=anc[a][i];
            b=anc[b][i];
        }
    }
    return anc[a][0];
}
int main() {
    int n=getint(),m=getint(),s=getint();
    for(int i=1;i<n;i++) add_edge(getint(),getint());
    dfs(s,0,0);
    while(m--) printf("%d
",LCA(getint(),getint()));
    return 0;
}