Submission #1196205

#	Time	Username	Problem	Language	Result	Execution time	Memory
1196205		countless	Duathlon (APIO18_duathlon)	C++20	0 / 100	1095 ms	6928 KiB

count_triplets

#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef long double ld;

const ll MOD = 998244353;
const ll INF = 1e18;
const ld EPS = 1e-12;

#define endl "\n"
#define sp <<" "<<
#define REP(i, a, b) for(ll i = a; i < b; i++)
#define dbg(x) cout << #x << " = " << x << endl
#define mp make_pair
#define pb push_back
#define fi first
#define se second
#define fast_io() ios_base::sync_with_stdio(false); cin.tie(NULL)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
#define sz(x) ((ll)(x).size())

struct custom_hash {
    static uint64_t splitmix64(uint64_t x) {
        // http://xorshift.di.unimi.it/splitmix64.c
        x += 0x9e3779b97f4a7c15;
        x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
        x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
        return x ^ (x >> 31);
    }

    size_t operator()(uint64_t x) const {
        static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
        return splitmix64(x + FIXED_RANDOM);
    }
};

template <typename Key, typename Value>
using hash_map = unordered_map<Key, Value, custom_hash>;

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
// uniform_int_distribution<int>(a, b)(rng);
// shuffle(all(a), rng);

ll end(int u, int p, const vector<vector<int>> &adj) {
    for (auto &v : adj[u]) {
        if (v != p) {
            return end(v, u, adj);
        }
    }
    return u;
}

ll size(int u, int p, const vector<vector<int>> &adj, vector<bool> &vis) {
    vis[u] = true;
    for (auto &v : adj[u]) {
        if (v != p) {
            return size(v, u, adj, vis) + 1;
        }
    }
    return 1;
}

ll find(int u, const vector<vector<int>> &adj, vector<bool> &vis, const vector<ll> &pref, int n) {
    ll st = end(u, -1, adj);
    ll ln = size(st, -1, adj, vis);
    if (ln <= 2) return 0;
    return 2LL * pref[n - 2];
}

void solution() {
    int n, m; cin >> n >> m;
    vector<vector<int>> adj(n);
    REP(i, 0, m) {
        int u, v; cin >> u >> v; --u, --v;
        adj[u].push_back(v);
        adj[v].push_back(u);
    }

    // subtask 3
    vector<ll> pref(n+1);
    iota(all(pref), 0);
    REP(i, 1, n+1) {
        pref[i] += pref[i-1];
    }
    
    REP(i, 1, n+1) {
        pref[i] += pref[i-1];
    }

    ll ans = 0;
    vector<bool> vis(n);

    REP(i, 0, n) {
        if (vis[i]) continue;
        ans += find(i, adj, vis, pref, n);
    }

    cout << ans << endl;
    return;
}

signed main() {
    fast_io();

    solution();

    return 0;
}

// Notes
// is it binsearchable?
// is it greedy?
// is it DP?
// if online round: search OEIS, similar topics

// put observations together
// check for edge cases
// try "dumb" solutions, you're fast enough to rewrite
// clean up the implementation
// keep it simple

// check for overflow
// check for small N
// check for array with same or monotonic a_i

• Subtask 1: 0 / 5
• Subtask 2: 0 / 11
• Subtask 3: 0 / 8
• Subtask 4: 0 / 10
• Subtask 5: 0 / 13
• Subtask 6: 0 / 15
• Subtask 7: 0 / 20
• Subtask 8: 0 / 8
• Subtask 9: 0 / 10