lca_tarjan.cpp

#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
#define pb push_back
#define mp make_pair
#define all(a) begin(a),end(a)
#define FOR(x,val,to) for(int x=(val);x<int((to));++x)
#define FORE(x,val,to) for(auto x=(val);x<=(to);++x)
#define FORR(x,arr) for(auto &x: arr)
#define FORS(x,plus,arr) for(auto x = begin(arr)+(plus); x != end(arr); ++x)
#define FORREV(x,plus,arr) for(auto x = (arr).rbegin()+(plus); x !=(arr).rend(); ++x)
#define REE(s_) {cout<<s_<<'\n';exit(0);}
#define GET(arr) for(auto &i: (arr)) sc(i)
#define whatis(x) cerr << #x << " is " << x << endl;
#define e1 first
#define e2 second
#define INF 0x7f7f7f7f
typedef std::pair<int,int> pi;
typedef std::vector<int> vi;
typedef std::vector<std::string> vs;
typedef int64_t ll;
typedef uint64_t ull;
#define umap unordered_map
#define uset unordered_set
using namespace std;
using namespace __gnu_pbds;

#ifdef _WIN32
#define getchar_unlocked() _getchar_nolock()
#define _CRT_DISABLE_PERFCRIT_LOCKS
#endif
template<class L, class R> ostream& operator<<(ostream &os, map<L, R> P) { for(auto const &vv: P)os<<"("<<vv.first<<","<<vv.second<<")"; return os; }
template<class T> ostream& operator<<(ostream &os, set<T> V) { os<<"[";for(auto const &vv:V)os<<vv<<","; os<<"]"; return os; }
template<class T> ostream& operator<<(ostream &os, vector<T> V) { os<<"[";for(auto const &vv:V)os<<vv<<","; os<<"]"; return os; }
template<class L, class R> ostream& operator<<(ostream &os, pair<L, R> P) { os<<"("<<P.first<<","<<P.second<<")"; return os; }
inline int fstoi(const string &str){auto it=str.begin();bool neg=0;int num=0;if(*it=='-')neg=1;else num=*it-'0';++it;while(it<str.end()) num=num*10+(*it++-'0');if(neg)num*=-1;return num;}
inline void getch(char &x){while(x = getchar_unlocked(), x < 33){;}}
inline void getstr(string &str){str.clear(); char cur;while(cur=getchar_unlocked(),cur<33){;}while(cur>32){str+=cur;cur=getchar_unlocked();}}
template<typename T> inline bool sc(T &num){ bool neg=0; int c; num=0; while(c=getchar_unlocked(),c<33){if(c == EOF) return false;} if(c=='-'){ neg=1; c=getchar_unlocked(); } for(;c>47;c=getchar_unlocked()) num=num*10+c-48; if(neg) num*=-1; return true;}template<typename T, typename ...Args> inline void sc(T &num, Args &...args){ bool neg=0; int c; num=0; while(c=getchar_unlocked(),c<33){;} if(c=='-'){ neg=1; c=getchar_unlocked(); } for(;c>47;c=getchar_unlocked()) num=num*10+c-48; if(neg) num*=-1; sc(args...); }
template<typename T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>; //s.find_by_order(), s.order_of_key() <- works like lower_bound
template<typename T> using ordered_map = tree<T, int, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
#define N 1000001

//offline fast;easy lca

int link[N],size[N];
vector<int> adj[N];
vector<int> queries[N];
bool vis[N];
int anc[N];
map<int,int> lca[N]; //May use a lot of memory, so to decrease that, change this into a pair vector and use sort+lower_bound

inline int find(int x){
    return x == link[x] ? x : link[x] = find(link[x]);
}

inline int unite(int fa, int fb){
    if(size[fa] < size[fb]) swap(fa,fb);
    size[fa] += size[fb];
    return link[fb] = fa;
}

int dfs(int v){
    vis[v] = 1;
    anc[v] = v;
    int fv = v;
    FORR(i,adj[v]){
        if(!vis[i]){
            int fi = dfs(i);
            anc[fv = unite(fi,fv)] = v;
        }
    }
    FORR(i,queries[v]){
        if(vis[i]){
            int mn = min(i,v);
            lca[mn][i+v-mn] = anc[find(i)];
        }
    }
    return fv;
}

int main(){
    ios_base::sync_with_stdio(0);cin.tie(0);
    int n;
    cin >> n;
    int cur;
    for(int i = 0; i < n; ++i){
        size[i] = 1;
        link[i] = i;
    }
    for(int i = 1; i < n; ++i){
        cin >> cur;
        adj[cur].pb(i);
    }
    int q;
    cin >> q;
    int f,s;
    vector<pair<int,int>> qu;
    while(q--){
        cin >> f >> s;
        queries[f].pb(s);
        queries[s].pb(f);
        int mn = min(f,s);
        qu.push_back({mn,f+s-mn});
    }
    dfs(0);
    FORR(i,qu){
        cout << lca[min(i.e1,i.e2)][max(i.e2,i.e1)] << '\n';
    }
}