toi19_explorer

#include <bits/stdc++.h>
using namespace std;
const int N = 5e5 + 3, mod = 1e9 + 7;
using ll = long long;
vector<int> adj[N];
ll dp[N], fac[N];

void dfs(int u, int p) {

    dp[u] = 1;
    int child = 0;
    for(int v : adj[u]) {
        if(v == p) continue ;
        dfs(v, u);
        dp[u] *= dp[v], dp[u] %= mod;
        child ++;
    }

    dp[u] *= fac[child], dp[u] %= mod;
}

int main() {
    cin.tie(nullptr)->sync_with_stdio(false);
    fac[0] = 1;
    for(int i=1; i<N; ++i) fac[i] = fac[i-1] * i, fac[i] %= mod;
    int n; cin >> n;
    unordered_set<int> s;
    int p = -1;
    for(int i=0; i<2*n-1; ++i) {
        int u; cin >> u;
        if(s.find(u) == s.end() and i) adj[u].emplace_back(p), adj[p].emplace_back(u);
        s.insert(u);
        p = u;
    }
    dfs(p, -1);
    cout << dp[p];
    return 0;
}