HIHO #1067 : 最近公共祖先·二（并查集+dfs LCA离线算法）

#include<cstdio>
#include<algorithm>
#include<iostream>
#include<cstring>
#include<string>
#include<cmath>
#include<queue>
#include<map>
#include<set>
#include<cstdlib>
#include<vector>
using namespace std;
#define cl(a,b) memset(a,b,sizeof(a))
#define LL long long
#define pb push_back
#define gcd __gcd

#define For(i,j,k) for(int i=(j);i<k;i++)
#define lowbit(i) (i&(-i))
#define _(x) printf("%d\n",x)

const int maxn = 2e6+10;
const int inf  = 1 << 28;

int id = 1;
map<string,int> mp;
map<int,string> tos;
int getid(string s){
if(!mp.count(s)){
mp[s]=id;
tos[id]=s;
return id++;
}
return mp[s];
}

struct node{
int index;
int x;
};
vector<node> query[maxn];
vector<int> G[maxn];

int f[maxn];
void init(int n){
for(int i=0;i<=n;i++){
f[i]=i;
}
}
int find(int x){
if(x==f[x])return x;
return f[x]=find(f[x]);
}
bool vis[maxn];
int ans[maxn];
void dfs(int u){
for(int i=0;i<G[u].size();i++){
int v = G[u][i];
dfs(v);
f[v]=u;
}
vis[u]=true;
for(int i=0;i<query[u].size();i++){
if(vis[query[u][i].x]){
ans[query[u][i].index] = find(query[u][i].x);
}
}
}

bool flag[maxn];// to get root of tree

int main(){
int n;scanf("%d",&n);

init(n);

for(int i=0;i<n;i++){
string a,b;cin>>a>>b;
int ida = getid(a);
int idb = getid(b);
flag[idb]=true;
G[ida].pb(idb);
}

int m;scanf("%d",&m);
for(int i=0;i<m;i++){
string a,b;cin>>a>>b;
int u = getid(a);
int v = getid(b);
query[u].pb((node){i,v});
query[v].pb((node){i,u});
}
int root = -1;
for(int i=1;i<id;i++)if(!flag[i]){root=i;break;}

dfs(root);

for(int i=0;i<m;i++){
cout<<tos[ans[i]]<<endl;
}
return 0;
}