hihocoder1067 最近公共祖先·二

思路：

使用tarjan算法，这是一种离线算法。

实现：

 #include <bits/stdc++.h>
 using namespace std;
 typedef pair<int, int> pii;
 const int MAXN = ;
 vector<int> G[MAXN], qry[MAXN];
 int par[MAXN], vis[MAXN];
 void init(int n)
 {
     for (int i = ; i <= n; i++) par[i] = i;
 }
 int find(int x)
 {
     if (par[x] == x) return par[x];
     return par[x] = find(par[x]);
 }
 int uni(int x, int y)
 {
     x = find(x); y = find(y);
     if (x == y) return x;
     par[x] = y; return y;
 }
 void dfs(int u, int f, map<pii, int> & ans)
 {
     vis[u] = ;
     for (auto it: qry[u])
     {
         int tmp = vis[it];
         if (!tmp) continue;
         if (tmp == )
             ans[pii(u, it)] = it;
         else if (tmp == )
             ans[pii(u, it)] = find(it);
     }
     for (int i = ; i < G[u].size(); i++)
         if (!vis[G[u][i]]) dfs(G[u][i], u, ans);
     vis[u] = ;
     uni(u, f);
 }
 int main()
 {
     int n, q;
     cin >> n;
     string x, y;
     map<string, int> mp;
     map<int, string> mp2;
     int id = , root = -;
     for (int i = ; i < n; i++)
     {
         cin >> x >> y;
         int pid = -, sid = -;
         if (!mp.count(x)) { mp[x] = pid = id; mp2[id] = x; id++; }
         else pid = mp[x];
         if (!mp.count(y)) { mp[y] = sid = id; mp2[id] = y; id++; }
         else sid = mp[y];
         if (i == ) root = pid;
         G[pid].push_back(sid);
     }
     cin >> q;
     vector<pii> v;
     for (int i = ; i < q; i++)
     {
         cin >> x >> y;
         int xid = mp[x], yid = mp[y];
         qry[xid].push_back(yid);
         qry[yid].push_back(xid);
         v.push_back(pii(xid, yid));
     }
     init(n);
     map<pii, int> ans;
     dfs(root, , ans);
     for (auto it: v)
     {
         int xid = it.first, yid = it.second;
         pii a(xid, yid), b(yid, xid);
         if (ans.count(a)) cout << mp2[ans[a]] << endl;
         else cout << mp2[ans[b]] << endl;
     }
     return ;
 }